Troubleshooting AO Smith Condensing Water Heaters and Error Codes Explained

Find out how to troubleshoot AO Smith condensing water heaters from 320 and 520 series using the error codes 101, 111, 121, 131, and others...

All condensing water heaters from AO Smith are self-diagnostic, so if there is a problem with the unit, a 3-digit error code (i.e., 101) will show up on the electric board or a remote controller (if installed).

This article is for those who have a problem with their tankless heater, received an error code, and have the tools, knowledge, and experience to deal with the problem. I always recommend contacting a professional and understanding that some people enjoy DIY projects, like to fix them, and save money.

This is the reason why we made this troubleshooting guide - to help you solve the problem with water heating and save some money.

Check if your water heater is properly installed

The guide will cover the most common problems found on tankless units, followed by the list of the error codes, causes, and possible solutions. Also, do not hesitate to contact AO Smith technical support for help.

I will assume that your tankless water heater was working correctly before the problem occurred. Whatever the problem is, make sure to check the following:

A water heater is connected to a 120 V grid and is adequately grounded, has unobstructed access to water and gas. The plumbing and gas lines are correctly sized, and the pressure is as required.

• Venting was done per code.
• The unit uses the correct gas type.
• The water heater is getting a sufficient air supply.

How does the AO Smith condensing water heater work

Before explaining the error codes, problems, and solutions, let's first see how condensing and tankless water heater works.

Before the tankless unit initiates gas ignition, a hot water tap has to be opened with water flowing at the rate of at least 0.5 gallons per minute.

• The fan starts.
• The igniter generates the spark.
• The gas valve opens.

The flame on the gas burner is lit, heating water in the heat exchanger. There is no storage tank in tankless heaters; water is just passing through the exchanger and further to the fixture. The red LED light on the computer board will display, indicating that the unit is ON.

Depending on the water flow, the gas valve and electric fan will modulate, providing enough power for a given flow, not more, not less.

The tankless stops heating as soon as the water fixture closes or when the demand for hot water stops.

Also, if, for any reason, water flow drops below 0.4 GPM, the water heater shuts down. The LED turns off while the fan continues to spin for another minute or two.

A list of error codes and troubleshooting tips

Troubleshooting AO Smith condensing water heaters is similar to troubleshooting other brands and models. Some of the problems are common, and they do not include an error code:

• Water is not hot enough or is too hot
• The temperature of the exiting water fluctuates
• Noise
• Burner does not ignite
• Long time to get hot water
• Sediment buildup

These and other common issues, including the cause and repair guide, can be found in the article about tankless problems.

Error code 031

Incorrect dipswitch setting - usually related to a gas type. Locate the switch on the computer board and check if it is in the correct position.

Error code 101

Improper gas combustion - check if there is an obstruction in the airflow; on the heat exchanger, the venting system, the fan, the burner, and then clean it. Abnormal gas combustion might also result from extreme weather conditions such as high winds or incorrect gas pressure.

Error code 111

Ignition failure - Check that the hi-limit switch hasn't tripped; the gas supply is correct; the flame sensor is clean; the ignitor generates sparks, and the venting system isn't blocked. Also, confirm that the PC board, overheat cut-off fuse, and gas valve are still operating. The gas line might need to be purged due to the presence of air.

Error code 121

Flame loss - the same causes as for error code 111.

Error code 311

Output thermistor failure (located on the heat exchanger) - check the wire connections (not loose, no burns, cuts...), clean the thermistor or replace if broken.

Error code 321

Inlet thermistor (located inside the water inlet) - check the wire, clean the thermistor or replace if broken.

Error code 331

Mixing thermistor (located in water outlet) failure - the same procedure as for the inlet and output thermistors. When testing the mixing outlet and inlet thermistors, the resistance value should be about 9.0 kOHM.

Error code 341

Exhaust thermistor (located above the heat exchanger) failure - The resistance value should be 10.75 kOHM.

Error code 391

Air-fuel ratio rod (AFR) failure - check the wire connections and clean the rod. Replace if needed.

Error code 441

Check the flow sensor for debris, clean the inlet filter, and ensure that the wires are not loose on the PC board.

Error code 510

Gas valve failure - check the gas pressure and connections.

Error code 551

Solenoid gas valve failure - check the gas pressure and connections.

Error code 611

Fan motor failure - check the wiring if it is loose or wet. Make sure to dry all the elements. Check the PC board; it might need replacement.

Error code 621

Exhaust fan motor issue - the same procedure as for error code 611.

Error code 631

Check the external pump connected to the PC board.

Error code 651

Flow adjustment valve failure - check the wire connections, PC board, and connection, water leakage from the valve, clean the valve, or if needed, replace it.

Error code 661

Faulty water bypass - the same procedure as for the flow adjustment valve.

Error code 701

The proportional gas valve doesn't work - check the wire connections and PC board. Check also the gas pressure supply.

Error code 711

Hi-limit switch / overheat cut-off fuse failure - inspect the connections and PC board.

Error code 721

If it is not a problem with the flame sensor, contact a professional or tech support.

Error code 741

There is no communication between the remote control and a water heater. Make sure that the controller is installed correctly.

Error code 761

Miscommunication between water heaters in Easy-Link setup.

Error code 941

Abnormal exhaust temperature - make sure that the vent length is per specs, there is no blockage in the venting pipes, the gas pressure is OK, and the exhaust fan motor operates correctly.

Error code 991

Improper gas combustion - see the error code 101.

You might want to reset the computer to clear the code for some error codes. When troubleshooting AO Smith condensing water heaters, for detailed instructions, use this link, which opens in PDF.

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