Navien Error Code E438 [Fixed A-Z]

The Navien error code E438 refers to improper operation of the circulation system. From what I have seen after working on many heaters over the years, it only appears in the Navien NPE A-type models. The most common reasons behind this issue are:

  • All water lines are not open.
  • Blocked cold water inlet filter.
  • Damaged flow sensor.
  • Faulty circulation pump.
  • Issues with the 2-way valve.
navien error code e438

Besides them, a faulty check valve and defective PCB can also cause this error code. I’ll explain how these issues can lead to this error code in detail along with their fixes. So, let’s dive in!

There are a few potential reasons that could lead to this issue. I’ll go through them along with their solutions one by one in this section.

01. All Water Lines Are Not Open

All the water lines must be open and working properly for the circulation system in your water heater to work well. This is especially important for the cold water supply line. If it’s not fully open, the required amount of water won’t go through the heater.

water lines of navien tankless water heater

As you know, tankless water heaters heat the water on demand. So, they need an adequate amount of water to go through the heat exchanger at the right speed. When this is interrupted due to any reason, enough water won’t circulate through the system and you’d see the dreaded E438 error code.

How To Fix:

If this is the issue, you’re in luck because this has a simple fix. Start by checking the main cold water supply valve first. Yes, I am talking about the one in your house.

If it’s already in a fully open position, check for kinks or obstructions in the water line. Any issues there can also lead to the heater not getting sufficient water and that can cause the recirculation issue.

Finally, check the cold water supply line and valve of the water heater itself. Make sure both valves are open in the water heater and the lines are secure. If all of them seem to be okay, then this is not the issue for you. Something else is causing the error code.

But if you notice issues with any of them, you have to fix them right away. If the main water supply line isn’t fully open, rectify that. Next, replace the water lines if there are obstructions in them so that there is a proper flow of water to the Navien water heater.

If the cold water supply valve in the water has been closed or not fully opened, you can easily adjust it and set it in the right position. If the lines have any debris in them, clean them to ensure proper flow of water. Once you implement these fixes, the error code should go away.

02. Blocked Cold Water Inlet Filter

Another common reason that I have seen cause the E438 error code in many Navien water heaters is an obstructed cold water inlet filter. This filter is a very important component in the water heater.

cold water inlet filter of navien tankless water heater

The water that flows through our houses is full of impurities. If all of these impurities passed through the heat exchanger, it would be damaged very quickly. The water filter prevents that from happening by stopping some of the impurities from getting into the heat exchanger.

Sadly, the water filter itself can be filled with impurities in that process. When that happens, the filter will get clogged. So, the water coming from the cold water supply line won’t be able to pass easily to the heater because of the obstructions in the water filter. So, the error code E438 can come up.

How To Fix:

Thankfully, this issue is also incredibly easy to fix. All you have to do is locate the water filter, inspect its condition, and clean it if it is dirty. When you do that, the error code should go away.

The filter is located at the bottom of the water heater. Look closely at the area where the cold water supply line meets the heater. That’s where you’ll find the cold water inlet filter. Here’s how it looks:

water filter location of navien tankless water heater

Next, inspect the condition of the water filter. Check if it’s actually dirty or not. You can turn off the cold water supply valve and remove the filter to inspect it properly. If it seems to be clean, the water filter isn’t the issue. But if it’s indeed filled with debris, you’ve just found the culprit.

Cleaning a dirty water filter is simple. You can rinse it in running water and that will be enough in most cases to clean it completely. However, if that doesn’t do the job, you can also use a toothbrush and soap to clean the cold water inlet filter.

Just make sure that you’re gentle when cleaning the filter. If you treat it too harshly, you may end up breaking it. In that case, your only option will be to replace it with a new one.

Attach the clean water filter to the water filter and turn on the cold water supply to the water. Hopefully, the error code will disappear and you’ll get hot water from your water heater once again.

03. Damaged Flow Sensor

Navien tankless water heaters are equipped with flow sensors. Their main job is to detect the flow of water through the system. When it senses water, it sends a signal to the control board to activate the heat exchanger.

So, the heat exchanger gets to work and you’re left with nice hot water at the other end. However, unfortunately, flow sensors can go bad with time and get damaged. In that case, it won’t be able to sense the water even when it’s flowing through the heater.

flow sensor of navien tankless water heater

It won’t signal the heat exchanger to get activated and you won’t get any hot water. It can also throw off the E438 error code as your water heater won’t know that there’s water. So, it can be thought that the circulation system is messed up when in reality, it’s an issue of the flow sensor.

You can test whether the flow sensor is bad by shutting off the cold water supply and turning it back on. When you turn on the water supply again, set the water heater to the internal recirculation mode.

Then, you have to check whether the flow sensor is getting accurate readings. To get to the reading of the flow sensor, you’ll have to press the “I” button on the keypad of your water heater. You can see some basic information about your water heater when you press that button.

The three types of information that you’ll see from that are the cold water inlet temperature, the hot water outlet temperature, and the GPM. But you’ll only see them one at a time. You can press the + and – buttons to switch between each of the information types.

Stop when you see the words “GPM” come up on the screen as that’s what you’ll need to test the flow sensor. It means Gallons Per Minute and that’s exactly the unit in which flow sensors measure the amount of incoming water.

If the flow sensor works properly, it should show you the proper reading due to the incoming cold water that goes through it. But if it’s faulty, it won’t show any reading as it won’t be able to sense the water passing through it. To get out of the information mode, press the reset button in the keypad.

How To Fix:

If the flow sensor in your Navien tankless water heater is damaged, you’ll have to replace it with a new one. It’s an easy job that shouldn’t take more than 15-20 minutes if you have a good grasp of doing maintenance work on your water heater.

How To Replace Navien Tankless Water Heater Flow Sensor?

You’ll only need a few basic tools like a pair of pliers and a Philips head screwdriver to do this job. The most important thing is to get the right flow sensor for your model.

There are different types of flow sensors out there. If you get the wrong one, won’t fit your water heater properly. So, you’ll need to get the right part to complete the replacement.

I’ve broken down the process into simple steps so that you can easily adhere to my instructions. Follow these steps to replace the flow sensor in your Navien tankless water heater:

Step 01 – Drain The Water Heater

Start by turning off the power to the unit to ensure your safety. Unplug the connection of the water heater from the power outlet as well. Shut off the cold water inlet and the hot water outlet valves.

Take a pair of pliers and open the service caps under both these valves so that you can start draining the water heater. You can place a small container close to the service caps as you remove them to collect the water that comes out of them. Doing them on both service caps may take a few minutes in total.

Remove the screws that hold the front cover of the water heater in place. Once you remove them, take off the cover and you’ll be able to look inside the heater. Your first job is to locate the flow sensor. Here’s what it looks like:

flow sensor location of navien tankless water heater

Step 02 – Remove The Flow Sensor Connections

You’ll see some cables that are connected to the flow sensor. I like to start by disconnecting the three-wire harness. Once you do that, remove the other wiring connection by simply pulling the connectors from one another.

The next step is to remove the stainless steel spring clip ring. Push it from the back of the water heater and then you’ll be able to get it out easily as it’s spring-loaded. Here’s where you should apply the pressure from:

flow sensor spring clip of navien tankless water heater

Then, you’ll have to remove the four screws that connect the flow sensor to the body of the water heater. Once you remove them, the flow sensor will no longer be attached to the heater. But you’ll still have to wiggle it a little bit to get it off the heater.

Step 03 – Remove The Flow Sensor

Make sure you’re gentle when you do it so that you don’t break anything. Once you get it a bit far from the heat exchanger, you can just twist the flow sensor and remove it from the water heater.

Before installing the new heater, compare the two flow sensors you have. Double-check that the new part looks exactly the same as the old part. If they’re not the same, make sure you get the right part first.

When you remove the old flow sensor, you’ll see that there’s an O-ring on the inside of it. Pull it out of that and slide it into the inlet pipe where the new flow sensor will be connected to it.

Step 04 – Install The Flow Sensor

Line up the new flow sensor carefully so that it matches the outline of where the screws will go. Once it makes a good seal, attach the four screws with a bit of forward pressure.

Then, reconnect the cold water inlet pipe to the bottom of the flow sensor by pushing the pipe into it. Push the sliding ring upwards so that it meets the flow sensor and reinstall the spring clip you removed earlier.

Connect the wiring connections you removed before. Make sure you connect them to the right connectors. Don’t worry about reversing them as they are keyed connectors and they can be plugged in backwards.

Step 05 – Check For Leaks And Final Touches

If any water comes out of the inlet pipe or when removing the flow sensor and lands in the body of the water heater, dry it using a paper towel. Then, you can close off both the hot and cold drain service valves.

Screw the caps back on beneath each of the service valves. Tighten them with a pair of pliers to make sure that water won’t leak through them. Then, you can open the hot water outlet and cold water inlet valves.

Open a hot water faucet to purge any air that may have gone into the system. Check the connections for leaks one more time. If there are any, tighten the loose connection to stop it. Put the front cover of the water heater back on and reconnect the screws of the water heater to secure it.

Finally, plug your water heater into the power outlet and turn it back on. Open a hot water faucet to create the demand for hot water and get the flow sensor to work. You should have hot water from now on.

Again, this is a very easy process. But if you don’t feel comfortable doing it yourself, feel free to hire an expert for this purpose. If the error code persists, something else is causing it.

04. Faulty Circulation Pump

If the flow sensor wasn’t causing the error code, there’s a very strong chance that it came up due to a faulty circulation pump. The Navien heaters in question are equipped with an internal circulation pump.

circulation pump of navien tankless water heater

It makes sure that there are very few fluctuations in the temperature when hot water is heated for use. If the circulation pump can’t detect the proper flow rate of the water, it won’t work. That’s how a faulty flow sensor can lead to the E438 error code.

However, the circulation pump itself can also be made and cause this error code. There are many ways to inspect whether the circulation pump in your water heater is working properly. The easiest one is to feel it by hand. If it vibrates when the heater is operational, it’s okay.

On top of that, there are a few more signs that you can use to determine whether the circulation pump is okay. Here are a few effects that you’ll notice with your water heater if the circulation pump goes bad:

  • The water heater itself can be frozen at times.
  • There are noticeable ups and downs in water temperatures when you set the water heater to the internal circulation mode.
  • When you set the water heater to external circulation mode, it would take longer than usual to get hot water at the hot water faucets.

You can also check the voltage of the connectors of the Navien water heater to test the circulation pump. When the water heater is turned off, it should read 0 volts. When it’s turned on, the reading can be from somewhere between 96 and 138 volts. If it’s outside that range, the circulation pump is faulty.

How To Fix:

By now, you should have all the tests on your Navien circulation pump and check whether it’s faulty or not. If it’s working well, you should move on to the next potential issue as they could also cause this error code.

However, if the tests reveal that you have a bad circulation pump, your only option is to replace it. Make sure you get the right part number for your water heater as there could be some variance in their shapes and sizes across all the Navien water heaters.

How To Replace Navien Water Heater Circulation Pump?

Replacing the circulation pump in your Navien water heater isn’t a difficult job. However, the process of doing that can be a little lengthy. I can confidently say that as I’ve done it myself. You can follow these steps to complete the Navien circulation pump replacement process:

Step 01 – Remove The Front Cover

Turn off the power to the water heater and shut off the gas supply as well. Disconnect the heater from the power outlet and turn off the water supplies as well. Shut off the cold water and hot water valves.

Drain the water heater by opening the service caps to remove the residual water in the unit. Then, remove the four screws at the top and the bottom of the unit to isolate the front cover of the heater.

Don’t worry about the access cover falling off. It’ll stick to the water heater even after you’re done removing the screws. You’ll have to wiggle it out to get it off the water heater.

Step 02 – Remove The Old Circulation Pump

Remove the wiring connection of the circulation pump to the control board. Remove the cold water inlet filter to drain some more water from the water heater. Take off the screws around it so that the circulation pump bracket gets loose and you can pull it out.

You’ll see a spring clip at the back of the circulation pump. You’ll have to pull it to loosen the circulation pump even more. Remove the two screws of the pump that hold it in place and pull it out carefully.

Step 03 – Install The New Circulation Pump

Place the O-ring of the new circulation pump at the point where it’ll meet the water heater. Then, put it in the position of the old circulation pump. Make sure the connections are sealed properly.

Attach the spring clip in its old position and tighten the screws of the pump to secure it in its place. Reinstall the parts that you removed previously and reattach the wiring connection of the circulation pump with the control board.

Put the front cover of the water heater back on. Restore the power supply, water supply, and gas supply to the water heater. Hopefully, it’ll work fine and the error code will no longer be a problem anymore.

05. 2-Way Valve Issues

By now, you must have realized that all the issues point to interruptions in the cold water supply in most cases. Another area where that mistake is possible is the 2-way valve.

First of all, if the 2-way valve isn’t facing the right direction, that can cause issues with the circulation pump. So, that can bring up the E438 error code.

For example, the 2-way valve has to be opened when you’re using the water heater in internal circulation mode. Similarly, it has to be closed when you use it in external circulation mode.

You can select the circulation mode of the water heater by setting the dip switches in the front panel. The dip switches are set to off by default. So, you’ll have to turn them on and then, adjust the position of the 2-way valve according to the circulation mode that you set.

The internal circulation mode in the Navien A series models is useful for the following reasons:

  • No need for any minimum flow rate to get hot water.
  • You won’t face any cold water sandwich issues.
  • The delivery of hot water to the faucets will be done in a shorter period.

However, keep in mind that you can’t get the use out of the 2-way valve if you don’t set it to the right position. It can also go bad with time. If that happens, you may also see the error code E438 pop up in your Navien heater.

How To Fix:

If the 2-way valve was set to the wrong position, you can easily adjust its position and see if that fixes the issue. Hopefully, you have a clear idea by now of how you should change its position depending on the circulation mode you’re using for the water heater.

But if the 2-way valve is faulty, setting it to the right position won’t make it okay. This could be the issue if your water heater works properly in either internal circulation mode or external circulation mode. But fails to work properly during both these modes.

Don’t worry, I won’t leave you on your own to take on the task of replacing the 2-way valve in your water heater. Here are the steps you can follow to replace the 2-way valve on your own:

Step 01 – Turn Off The Water Heater

Turn off the power supply to the unit and unplug it from the power outlet. Shut off the gas supply and the water supply as well. Drain the unit by opening the service caps as you’ll be working inside the heater and you don’t want any water spillage during that time.

Step 02 – Loosen The 2-Way Valve

Remove the screws at the top and bottom of the heater that holds the access cover in place. Usually, you’ll find two screws at the top and two at the bottom. Take out the access cover after removing the screws.

The 2-way valve is connected to the hot water side of the water heater. Look for the three screws at the hot water outlet fitting and remove them to loosen the 2-way valve.

There will be four stainless steel clips connected to the 2-way valve. Usually, two of them will be round and two of them will be flat. You’ll have to pull them and remove them to loosen the 2-way valve.

Step 03 – Remove The 2-Way Valve

Once the 2-way valve comes loose, you can remove the wiring connector at the 2-way valve. To completely remove the 2-way valve, you’ll have to go through the buffer tank. So, you’ll have to remove the buffer tank first.

The buffer tank will be connected to the internal body of the water heater with three screws. Remove them and then, you can easily pull out the buffer tank. When you do that, you can easily remove the 2-way valve.

Step 04 – Replace The 2-Way Valve

Install the new 2-way valve in place of the old one and reattach the buffer tank. Tighten the screws of the buffer tank to secure it in its original position. Place the stainless steel clips you removed earlier to where they were earlier. Reattach the wiring connector of the 2-way valve.

Next, put the front cover of the water heater where it belongs and tighten the screws. Then, you can restore the power supply of the water heater. Make sure you open the gas and water supply as well. Open a hot water faucet and check for leaks.

If there are leaks, you need to tighten the connections from where the water heater is leaking. If there is no leaking, well done – you did a great job. That’s how you can easily replace the 2-way valve on your own. As you’ve noticed, it doesn’t take any technical expertise to do it.

However, if you’re not comfortable doing it yourself, you can always hire a qualified expert to do the job. Hopefully, the error code will go away after replacing the faulty 2-way valve. If the code still remains, then please carry on reading this guide to find the right solution.

06. Check Valve

The check valve is another component that can also lead to the Navien E438 error code. It plays a key role in preventing the backflow of water. In the Navein A series models, an in-built check valve is a part of the system.

The check valve makes sure that the hot water doesn’t get mixed with the cold water when they are circulated within the water heater. So, when something goes wrong with it, the hot water gets mixed with the cold water. That’s what causes changes in water temperature and leads to the error code.

navien check valve of navien tankless water heater

The most common symptom of a faulty check valve is extreme fluctuations in temperature. For instance, you may get hot water when you turn on the water heater but it switches to cold water after that. There could even be fluctuations of 10-15 degrees when you have a bad check valve.

Another way to inspect the check valve is to check the hot water temperature. You can do it by pressing the “I” button in your water heater to access the information mode of the water heater.

You’ll be able to check the hot water temperature when you do that. If it gives a low reading like 70 or 80 degrees Fahrenheit, there’s a strong likelihood that you have a faulty check valve.

How To Fix:

When you have a faulty check valve, the permanent solution is to replace it. However, if you’re early in the diagnostic process and you don’t have a replacement part yet, there may be a temporary solution for you.

Notice that I used the word “maybe” because it may not work all the time. You’ll need around 12 ounces of vinegar to implement it. Boil some water and mix it with the vinegar to make this vinegar solution.

Remove the check valve for your unit and submerge it in the vinegar solution for a few hours. It may be enough to remove the build-up of impurities in the check valve. If it works, you can continue to use your water heater until the new check valve arrives.

Unfortunately, if it doesn’t work, you’ll just have to wait for the replacement part to arrive before you can use the water heater again. Don’t worry if you don’t know how to remove the check valve. I’ll talk about its location and replacement process in detail in the next sections.

Where Is The Navien Water Heater Check Valve Located?

Before replacing the check valve, you need to know its location. As you’ve come to this guide looking for a solution for the E438 error code, I’ll assume that you have a Navien A series model unit.

The special thing about these units is that they have an internal check valve. As the check valve is internal, you’ll find it inside the unit when you remove the front cover of the water heater.

When you use external check valves, you have to set the return lines outside the heater. In that case, the check valve will be installed outside the heater somewhere else in the water lines.

If you have an internal check valve in your Navien water heater, you may be wondering where exactly it is located. You’ll notice it behind the keypad of your water heater. Here’s a picture to help you out:

check valve location of navien tankless water heater

How To Replace Navien Tankless Water Heater Check Valve?

Replacing the Navien check valve is one of the easiest jobs you can possibly do in your water heater. All you’ll need is a pair of needle nose pliers and the right replacement check valve to do it completely on your own.

First, you’ll have to turn off the water heater and remove the front cover to access the check valve. You already know where the check valve is located from the last section. All you have to do is to grab it with the needle nose pliers. Here’s what it’ll look like once you’ve removed the check valve:

check valve of navien tankless water heater

Then, you have to turn it counterclockwise. Don’t worry, it’s not even a quarter of a turn. When you do that, give it a push and you’ll be able to wiggle it right out. That’s literally how easy it is to remove the check valve.

Next, put the new check valve in place of the old one using the same pliers. You’ll have to align it with the notches to be able to push it in. Then, you can turn it clockwise to seal it in its place.

There you have it – that’s how easy it is to replace the check valve. Remember that prevention is always better than cure when it comes to tankless water heaters. You could have easily avoided these issues if you took good care of your water heater over the years.

Hopefully, the error code will go away after replacing the check valve. The most important step in maintaining your water heater is to flush the water heater at the right time. Doing that would have stopped the build-up of scale and made sure that your water heater works in good condition.

I’ve already written a dedicated guide on flushing a Navien tankless water heater. Check it out and make sure you follow it to keep these issues at bay. Apart from that, I also have a Navien maintenance guide that you can follow to know everything you need to keep the heater healthy in the long run.

07. Defective PCB

If the error code still remains even after implementing all these solutions, there is one more thing that you should try. And that is the PCB or the control board of your Navien tankless water heater.

PCB of navien tankless water heater

At this stage, you’ve really tried all the solutions that you could. If you’ve checked all the things I mentioned so far, you may have found that they are all working fine. That means they can’t lead to the E438 error code.

So, the final possibility is that the PCB is defective. The components in your water heater work with the PCB to make sure the heater works properly. That includes the flow sensor, circulation pump, check valve, 2-way valve, etc.

If the flow sensor and the valves work well, the flow of water won’t be obstructed. But if the control board doesn’t get that signal, it can seem like other parts are at fault. In some of those cases, the error code E438 appears.

How To Fix:

You’ll have to replace the PCB if it’s faulty. That’s the only way you can get your water heater to work again. When you do that, you’ll have done everything you can to solve this error code.

How To Replace Navien Tankless Water Heater PCB?

Replacing the PCB may seem like a daunting task. But don’t worry, you’re in good hands as I’ve done this replacement job many times. I’ll walk you through the entire replacement process. Here are the steps you need to follow to replace the PCB in your water heater:

Step 01 – Take Off The Front Cover

As always, you have to follow the safety protocols when replacing any component of the water heater. So, that means you have to turn off the power supply, gas supply, and water supply of the water heater. Unplug the connection from the power outlet as well.

Then, remove the front cover of the water heater after undoing the screws that it in place. Before you do anything else, match the part number of the faulty PCB with the new replacement part.

Make sure you’ve got the right component for your water heater. If not, wait until the proper part arrives. Next, take a picture of the PCB in its current condition. It’ll help you remember the several wiring connections and pin connectors of the water heater.

Pro Tip: Use painter’s tape and mark some numbers on them to keep track of the wires you removed. Some of them look the same and it’s important that you don’t get lost when you remove them.

Step 02 – Remove The Connections To The PCB

In some models, you’ll have to replace the display unit to get to the PCB. If that’s what happens in your case, just do that. Otherwise, you should have direct access when you remove the front cover.

There will be many wiring connections right at the PCB. You have to remove them one by one before you can remove the PCB. As you disconnect each of these wires, mark them with the tape so that you don’t make a mistake when you reconnect them later on.

Some of the wiring connections will be harder to remove than others. You need to use a pair of pliers to loosen them. Then, you can pull them out easily. I like to keep things organized.

So, I’d suggest you start with the left-hand side. When you get to the bottom, the connections are for leak detectors. So, make sure you remove them as well. Similarly, remove the connections at the right-hand side.

Next, you’ll have to remove the mounting screws that hold the control board in place. Two of them are also located at the bottom of the water heater. When you remove them, the PCB will come loose.

The PCB won’t fall because it’ll still be connected to a wiring harness in the back. It connects to the power switch of your heater. When you pull out that wiring harness, you’ll be able to pull out the PCB completely from the heater.

Step 03 – Install New PCB And Tighten The Connections

At this point, it’s essentially a process of reverse-engineering the previous steps. Connect the wiring harness of the water to the new PCB and put it in place of the old one. Then, tighten the screws to hold it in place. Initially, you can just hand-tighten them.

Then, reconnect the wiring connections that you removed from the water heater. This is where the painter’s tape and the picture you took earlier will come in handy. Follow them to reconnect everything correctly.

Once you do that, you can attach the front cover of the water heater back on and restore the power supply of the water heater. Turn on the gas and water supply and see if it works properly now.

That’s how simple it is to replace the PCB. However, if you think it’s hard, don’t hesitate to hire a professional to do this job. Hopefully, the error code will go away and you’ll no longer have any issues with your water heater.

How To Reset Navien Tankless Water Heater?

The error codes may not automatically go away when you implement the fixes that cause the error code. If it remains even after you’ve followed through on the solution, a good idea is to reset the water heater.

Thankfully, resetting Navien tankless water heaters is a piece of cake. All you have to do is press the reset button and the water heater will reset itself. The reset process will be done automatically by the heater when you press the button. 

However, if it doesn’t go smoothly, I have a solution for you. Check out my Navien water heater reset guide if you’re having any issues resetting with the heater. You’ll also learn how to hard reset the water heater in there.

If the error code still remains after you’ve implemented the solutions and reset the water heater, it’s time you bring in an expert it’s time you bring in an expert or contact Navien support. You’ve tried everything you could and now you have no other option but to bring in a professional. Hopefully, he’ll be able to guide you and fix the issue.


Is flushing the Navien tankless water heater important?

When you flush the water heater, the different components of the water heater get cleaned. Especially, the heat exchanger as most scale builds up there. Flushing ensures that your heater stays in good condition over a long period.

What does the blue cable connected to the flow sensor do?

It helps the flow sensor detect the temperature of the water that’s coming in the heater. It’ll determine how much the heat exchanger needs to be engaged to get the water to the desired temperature.

How do you clean the air intake filter?

You have to unscrew the air intake filter from the top part of the heater and pull it out. Then, you can use a toothbrush and soapy water to get the dirt out of it.

What is the right flow of water when using the internal circulation mode?

If you have a Navien A-type water heater and you’ve activated the internal circulation mode, the flow rate should be close to 3 GPM. If the pump works but you see a 0.0 GPM flow rate reading, the flow sensor is defective.

Can the flow sensor get dirty with time?

The flow sensor sits on the cold water inlet line right where it goes into the heat exchanger. So, if there are impurities in the water, it’ll also get dirty with time just like the heat exchanger.


Now, you know what causes the Navien error code E438 and how you can fix it. The most important thing is to be patient when you make your diagnosis so that you can find the right culprit. Hopefully, it’ll be an easy fix like the water lines not being open for you.

However, you’re ready to deal with all the possible issues that can lead to this error code. I’ve also compiled detailed solutions for other Navien error codes. Check out my guide on solving Navien error code E110 if it comes up.

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