AO Smith Tankless Water Heater Troubleshooting – Your Complete Guide 2021

Are you looking for a detailed guide on AO Smith tankless water heater troubleshooting? Look no further.

Our in-depth article covers condensing, non-condensing, indoor, outdoor, and low NOx water heaters with cutting-edge technology.

Have you ever noticed what happens when you activate the hot water tap and all you acquire is chilled water?

Probably you go to wash utensils and find your AO Smith hot water heater not working.

Not having immediate ingress to warm water is infuriating and annoying.

If your device is displaying a mysterious code and you cannot interpret what it means, don’t panic—we are here to help you understand those enigmatic error codes and troubleshoot your system without requiring professional assistance.   

Where Can You Find The Error Codes On The AO Smith Tankless Water Heater?

AO Smith Tankless Water Heater Troubleshooting

AO Smith water heaters are furnished with a remote controller. This self-diagnostic function helps in detecting the problem.

If the machine is not functioning properly, the remote will make a sound, and the error code will appear on the display screen.

It is critical to supply the device with power and an accurate connection to have the device operating efficiently.

You can troubleshoot the issues yourself, but we would recommend you hire expert troubleshooting services.

If the remote controller is not deployed on the unit, and there is something wrong, the code will be displayed differently.

For instance, the 110 and 310 models will display a flashing red LED that will blink one or more times depending on the error, whereas the 510 model will show the code on the display screen.

On the flip side, the models 240, 340, and 540 will display the code on both the remote and temperature controllers.

It is better to check the AO Smith tankless water heater installation manual to determine what model you have and where to look for the numerical code.  

Tools Required For AO Smith Tankless Water Heater Repair

  • Screwdriver
  • Manometer
  • Multi-meter

AO Smith Hot Water Heater Troubleshooting

We will assume that your appliance, before the issue occurred, was accurately deployed according to codes and instructions of the brand.

It was attached to a 120-volt grid, was sufficiently grounded, and had unblocked access to gas and water.

The plumbing and gas lines were accurately sized, and the pressure was according to the requirements.

The venting was done according to the code, the correct type was utilized, and the device obtained an adequate air supply.  

We will show three error codes, the first for the heaters that are without a remote controller and have only blinking LED, the second for the machines that have remote controller, and the third for the units that have 7-SEG LED.

How Does The AO Smith Water Heater Operate?

Heat pump numbers@2x

Before discussing the AO Smith tankless water heater problems and how to reset AO Smith Promax water heater, let’s see how the water heater operates.

Before the unit starts gas ignition, you have to open the tap with water flowing at the rate of 0.5 GPM.

After that, the fan initiates, the igniter produces the flame, and the gas valve opens.

The spark on the gas burner is ignited, warming water in the heat exchanger.

Tankless units don’t have tanks to store water, so the water flows through the exchanger and to the hot water tap.

The red LED light on the computer board will show to inform you the unit is on.

The electric fan and gas valve will adjust to deliver sufficient power for a given flow depending on the water flow rate.

The unit stops heating water when you turn off the tap.

Besides, if the water flow rate drops from 0.4 gallons per minute, the device stops working.

The red LED light shuts off while the electric fan carries on to rotate for another 2 to 3 minutes.

AO Smith Tankless Water Heater Problems And Troubleshooting

AO Smith Tankless Water Heater Troubleshooting

Error Code: Remote 03, LED 031, Light Blinks One Time

Inappropriate dipswitch setting.

  • This issue usually occurs in gas devices.
  • Shut off the power supply.
  • Pinpoint the switch on the computer board and make sure the settings are according to the user manual.

Error Code: Remote 10, LED Display 101, Light Blinks Five Times

It is indicating the incorrect gas combustion.

  • Check for obstructions such as bird nests or leaves in the vents.
  • Intake and exhaust terminals must have accurate clearance between them.
  • Examine the fan and burner for dirt and grime, and clean if required.
  • Extreme weather conditions such as storms or improper gas pressure could cause abnormal gas combustion.

Error Code: Remote 11, LED Display 111, Light Blinks Three Times

This numerical code is telling you about the ignition failure.

  • Before AO Smith tankless water heater troubleshooting, test the hi-limit switch and confirm it is working fine and the wire connections are not loose and broken.
  • The flame rod should be free from dirt and grease, and the computer board should not be damaged.
  • You should be able to listen to two thump sounds from the gas valve or a bustling spark ignition from the gas burner.

Error Code: Remote 12, LED Display 121, Light Blinks Three Times

This code means there is flame loss during the operation.

  • Check if the hi-limit switch has not tripped.
  • Confirm the settings of the computer board are accurate, according to the user manual.
  • There should not be any obstructions in the gas supply.
  • The gas should be on and at the correct pressure.
  • Also, make sure there are no blockages in the vents.

Error Code: Remote 31, LED Display 311, Light Blinks Two Times

This code tells you about the output thermistor failure.

  • Verify there are no cuts and burns on the wire connections and also they are not loose.
  • Check the thermistor and change if damaged.

Error Code: Remote 32, LED Display 321, Light Blinks Two Times

It is informing you about the failure of the inlet thermistor.

  • Check the thermistor wiring and ensure connections are good and not loose or broken.
  • Then, check the thermistor for dirt and grime, and clean or replace it if required.

Error Code: 331, Light Blinks Two Times

It lets you know about the failure of the mixing thermistor.

  • Check the thermistor for dirt or debris and loose, damaged, or broken wire connections.
  • Clean the mixing thermistor or replace it if necessary.

Error Code: 341, Light Blinks Two Times

It means failure of exhaust thermistor.

  • Examine the wire connections of the thermistor and make sure they are good and not damaged.
  • Check the thermistor for dirt and clean if required.
  • Make sure the resistance valve is 10.75 Kohm.
  • If the issue persists, change the exhaust thermistor.

Error Code: Remote 39, LED Display 391, Light Blinks Two Times

It means the air-fuel ratio rod has failed.

  • Verify the wire connections are not broken or loose and clean the rod.
  • If the issue persists, replace the rod.

Error Code: 441, Light Blinks Two Times

It indicates that there is something wrong with the flow sensor.

  • Test the wire connection and make sure it is not loose or broken.
  • Besides, the flow sensor impeller should be free from debris.

Error Code: Remote 51, 55, LED Display 510, 511, Light Blinks Six Times

There is some issue with the gas solenoid valve.

  • Confirm the wires of the gas valve assembly are not cracked or broken before AO Smith tankless water heater troubleshooting.
  • Also, make sure the gas pressure is at the correct level.

Error Code: Remote 61, LED Display 611, Light Blinks Four Times

The fan motor is defective.

  • Make sure the wire connections are not wet or loose. All the elements must be dry.
  • Also, inspect the PC board. If the problem lasts, replace the PC board.

Error Code: 651, Light Blinks Four Times

This error code is telling you about the flow adjustment valve fault.

  • Inspect the flow adjustment valve wiring for damages or breakages.
  • The valve should not be leaked.
  • Also, look for any accumulation on the valve and clean it if necessary.
  • If it is not possible to clean it, replace the valve.

Error Code: Remote 74, LED Display 741 And 751

It means there is miscommunication between the unit and the remote controller. It happens if you detach the remote from the appliance while the power is on.

  • Shut off the power before detaching the remote from the heater.
  • Check the connection between the device and remote and confirm you have the right remote.
  • Check there are not burnt or damaged wires on the control board.
  • A little surge or outage can cause the display screen to flash where power has been absent.

Error Code: Remote 99, LED Display 991, Light Blinks Five Times

It is informing you about improper gas combustion.

  • Guarantee the machine is set for the right elevation.
  • Also, make sure there are no obstructions in the venting system, and the distance between the intake and exhaust is not too small.
  • Also, look for external particles or grease on the fan and burner.
  • The surrounding air should not be polluted.


Now you know about error codes and AO Smith tankless water heater troubleshooting, you can initiate fixing any issues on your own.
However, if you are unsure how to fix the problem, call an expert technician.
We hope you found our article helpful and enlightening.
If you have any tips or questions, feel free to reach out to us, and don’t forget to share it with your friends who own an AO Smith heater.

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