Error Codes and Troubleshooting AO Smith Condensing WH

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How to troubleshoot AO Smith condensing water heaters from 320 and 520 series and understanding the error codes 101, 111, 121, 131 and other...

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 computer board or remote control (if it is installed). This article is for those who have a problem with the heater, received an error code and are willing to deal with the problem. I always recommend calling the professional, but understand that some people will give a try first. 

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

Is your water heater properly installed?

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

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

  • Water heater is connected to 120 V grid and is properly grounded, has an unobstructed access to water and gas. The plumbing and gas lines are correctly sized, and the pressure is as required.

  • Venting was done per codes.
  • The unit uses the correct gas type.

  • The water heater is getting sufficient air supply.

How AO Smith condensing water heater work

  • Hot water tap is open, and the flow is greater than 0.5 gallons per minute, which is a requirement for the activation.

  • The fan starts.

  • The igniter generates the spark.

  • The gas valve opens.

  • The flame is lit and the red LED light on the computer board will display.

  • The gas valve and electric fan will modulate based on the hot water flow (demand).

If, for any reason, water flow drops below 0.4 GPM the water heater will shut down. The LED will turn off and the fan continuous to spin for another minute or two.

Error codes and Troubleshooting

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.

  • 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 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 that it is in the correct position.

Error code 101

Improper gas combustion - check if there is an obstruction in the air flow; on the heat exchanger, venting system, fan and burner and clean it. Abnormal gas combustion might also be a result of the extreme weather conditions such as high wind or incorrect gas pressure.

Error code 111

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

emergency plumber

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 make sure 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 - the same as the above.

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 e.c. 611.

Error code 631

Check the external pump connection to the PC board.

Error code 651

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

Error code 661

Faulty water bypass - the same procedure as for 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 cutoff fuse failure - inspect the connections and PC board.

Error code 721

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

Error code 741

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

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 exhaust fan motor operates correctly.

Error code 991

Improper gas combustion - see the e.c. 101.

Keep in mind that for some error codes, you might want to reset the computer to clear the code. For detail instructions when troubleshooting AO Smith condensing water heaters use this link, which opens in PDF.


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