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AO Smith Tankless Error Codes: Problems And Troubleshooting

Note: This post may contain affiliate links. This means that at no cost to you, we may receive a small commission for made purchases.

If you own an AO Smith tankless water heater, then it’s important that you know how it works and what to do when things go wrong.

However, sometimes the provided manuals and instruction guides are not very clear and if you need help addressing some error codes that are popping up on your AO Smith’s tankless water heater, then we have got you covered. 

We are going to cover some of the most common error codes and problems seen with the AO Smith tankless water heater so you know what you can do when the issue occurs.

Check out the guide below for easy to understand explanations about each error code and the troubleshooting tips that can help you solve the issues you are facing with your AO Smith tankless water heater.  

It’s important to note that the error codes we will be addressing in this guide are found on the 110, 510, 310, 240, 240 and 540 AO Smith tankless water heater models.

Any other models may have different codes for different issues. 

So, let’s jump in! 

Error Code 03 (031)

If this error code has appeared on your AO Smith tankless water heater’s LED screen, then this is a sign that you have set the incorrect settings for the dipswitch.

Another sign that you are facing the same problem would be if the LED light is blinking once. 

To solve this problem, you will need to find the correct settings for your water heater’s model and reset them on the computer board.

You should be able to find the settings in your water heater’s manual, but if you are stuck, contact the support at AO Smith.

They should be able to tell you the correct dipswitch settings for your model so you can correct it yourself. 

Error Code 10 (101)

This error code is usually a sign that there is some kind of blockage in your vents, gas burner or fan motor. Another sign that this is the issue is if the error code is blinking five times. 

To solve this, you will need to unblock your vents and also check for any grease or dirt found within the gas burner or fan motor.

This debris will be what is preventing your water heater from functioning properly. Before you start to disassemble your water heater, remember to shut off its power and that you reassemble it correctly. 

Error Code 11 (111)

This is a popular problem that a lot of AO Smith tankless water heater owners suffer from because this error code usually occurs when there is an ignition failure. The error code may also be flashing three times. 

Ignition failure is not an uncommon issue with water heaters and boilers, but it’s also highly recommended that you solve this issue by calling a technician to your home.

You can do a few basic checks first – like making sure there are no loose wire connections or soot on the flame rod.

If everything seems to be in order but your water heater is still failing to ignite, then it is safer to call out a technician to take a look at it. 

Error Code 12 (121)

This error code is closely related to the error code 11 (111) as it is a sign that your water heater is losing its flame.

While it may be igniting, it’s not keeping its flame and is going out before it can adequately warm up your water. 

Again, this is an issue for a professional – but you can check that the proper settings are in place on the computer board.

Double check with your water heater’s manual first and if everything seems to be fine, then you need to call out a technician. 

Error Code 31 (311)

Thermistors are an important component of any water heater as it helps your water heater heat your water to the correct temperature.

There are actually many types of thermistors in your water heater – and thus, any error codes that relate to problems they are facing. 

This error code signifies that there is a problem with your outlet thermistor. This is the thermistor attached to your outlet pipes.

To solve this error, check the thermistor to see if there are any loose wires or breakages that are causing the failure. Debris and dirt could also be affecting the thermistor, so clean anything away and try again. 

Error Code 32 (321)

This error code occurs when there is a problem with the inlet thermistor. It functions just like an outlet thermistor, but is found on your inlet pipes instead of your outlet pipes. 

The solution to this problem is just the same as the previous error code – check for broken wires, damage, loose connections, dirt and debris.

Once your inlet thermistor is clean and properly connected, then the error code should vanish and your water heater return to its usual state. 

Error Code 331

This error code is another thermistor failure but this time, it is referring to the mixing or mix thermistor.

Where this thermistor is will vary from model to model, so you will need to check your manual to find it on your water heater. 

Once you have, make sure all the connections are secure and safe and clean away any dirt or debris that could be the issue. Once that’s one, the error code should vanish. 

Error Code 341 

This is the final error code for thermistor failure. This time, it is referring to a problem with your exhaust thermistor. This thermistor should be found attached to your water heater’s exhaust. 

Just like with the previous thermistor failure errors, you should follow the same process to solve it.

Clean away any dirt or debris and check the quality of the connection wires. You should also ensure they are securely attached. 

Error Code 39 (391)

Error Code 39 (391)

If your display screen is showing this error code and is flashing twice, then this is a sign that there is a problem with your water heater’s air-fuel ratio rod (sometimes referred to as the AFR for short).

AFR rods monitor the air fuel ratio of your water heater’s internal combustion engine, which is a massively important part of your water heater’s function.

However, damage and soot can sometimes prevent the AFR rod from doing its job properly and it requires attention. 

It’s super important that you keep your AFR rod in the best condition so you can keep using your water heater safely.

Clean your AFR rod of any soot, and check for any damage or loose connections to see if you need to find a replacement.

Sometimes if you are lucky, it could just be a bit of soot blocking the sensors – other times, you may need to find a replacement AFR rod as soon as possible. 

Error Code 441

This error code indicates that there is a problem with the flow sensor on your AO smith tankless water heater. 

Most of the time, this error occurs when there is a wire connection issue or a breakage, which can easily be fixed or replaced.

Alternatively, it could be that debris has affected the flow sensor’s impeller and just needs a bit of cleaning to get back to functioning as normal. 

Error Code 51 Or 55, 510 Or 551

All of the above error codes all point to the same problem: an issue with your gas solenoid valve. 

This error should really be addressed by a technician, but you can also open up your water heater to check for any wire breakages or loose connections. 

Error Code 61 (611)

This error code pops up whenever there is a fault with the fan motor of your tankless water heater. Alternatively, the error code will flash four times. 

The cause of this problem could be a range of different issues including connection issues, broken wires, and even corrosion around the connectors.

If you have recently experienced cold weather, it could also be that your connectors are frozen and need warming up.

The fan motor could also be blocked by dust, debris or dirt and needs a clean. Also, you should check the computer board for any sign of burn marks too. 

Error Code 651

This error code usually means there is a fault with the flow adjustment valve. 

This is usually caused by scale build up which locks the motor drive. Scale is caused by water settling on heat transfer surfaces and the impurities build up into a hard, crusty layer.

If you see any scale buildup on your motor drive, you will need to clean it away with hydrochloric acid and water mixture (wear appropriate protection gear while handling this mixture).

If there is a scale build up, then you should also check for any water leakages that are causing scale to build up inside your water heater. 

If there is no scale build up, then you should check the connections and wires around the flow adjustment valve. 

Error Code 661 (540 Model Only)

If you own a 540 model AO Smith tankless water heater, then this is an error code that may appear on your display screen. It usually means that there is a problem with the bypass valve. 

To solve this error,  you will need to carry out the same checks and treatments in  the above error.

Scale build up could be the cause of locking the motor drive and is usually caused by a water leak somewhere within your water heater.

If there is no scale, then you should also check the connections and wires for damage or looseness. 

Error Code 70 (701)

This error code indicates that there is a problem with your water heater’s computer board. 

There are a range of potential causes to this error. There could be an issue with the power supply of your water heater and thus it could be failing.

Alternatively, you should check for any loose connections and damaged wires that could be behind this power loss.

If there are any burn marks on the computer board then this is a sign that your water heater’s computer board is overheating and cutting out. 

Error Code 72 (721)

This final error code indicates that there is a false flame being detected within your water heater. 

To solve this issue, you need to check for any condensation within the heater.

If the condensate drain is not properly installed on the vent collar of the water heater, then there could be water in the internal combustion engine.


We hope that this article has provided you with some useful information about AO Smith tankless errors codes. Now, you should be able to understand how these error codes work and what they mean.

You can now use this knowledge to help diagnose and fix problems with your AO Smith tankless water heater so you can continue enjoying hot water with little fuss! 

author avatar
Anthony Barnes
Anthony Barnes is the founder of Water Heater Hub and a second-generation plumber by profession. Before developing Water Heater Hub, Anthony Barnes was a full-time plumber, and he has undertaken a wide variety of projects over the decades. As a second-generation plumber, it was easy for Anthony to get used to the technicalities of all from a tender age


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