AO Smith tankless water heater error codes typically point to specific issues like ignition failures (e.g., 111, 121, 991), sensor problems (e.g., 03, 05, EEE), or other component malfunctions.

To troubleshoot, first try resetting the unit by turning the power off and on, then check for obvious issues such as gas supply or vent blockages. For more complex problems, you may need to clean components or consult SPS Plumbing for more help.

IMPORTANT SAFETY NOTE:

If you have any concerns about your own or someone else’s safety, please stop troubleshooting and call SPS Plumbing immediately.

Error Code Reference

Error Code(s)

111, 121, 991

Issue

Ignition failure, flame failure, or vent blockage

Solution

Check and open the gas supply valve. Clean the igniter and flame sensor. Inspect and clear any debris from the exhaust vents.

Error Code(s)

03

Issue

Heating element or sensor issue

Solution

Turn off power and check heating elements for continuity with a multimeter. Inspect sensor wiring for loose connections or damage.

Error Code(s)

05

Issue

Temperature sensor/thermistor fault

Solution

Turn off power and inspect sensor wiring. If wiring is intact, test the thermistor's resistance.

Error Code(s)

72

Issue

False flame detection

Solution

Clean the flame rod. Check for leaks from the heat exchanger and ensure the condensate drain is installed correctly for indoor models.

Error Code(s)

611, 621

Issue

Fan motor or exhaust fan motor fault

Solution

Check for loose wires, dust buildup, or corrosion in the fan motor. Check for burn marks on the control board.

Error Code(s)

33, 34, 52, 61

Issue

Various sensor or fan motor issues

Solution

These codes can indicate problems with the outgoing temperature sensor, combustion air temperature sensor, modulating gas solenoid valve, or combustion fan. Check wiring, sensor resistance, and fan operation.

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