AO Smith Tankless Error Codes
Troubleshooting Guide
(updated 2017)


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Learn about AO Smith tankless error codes for an easy and inexpensive troubleshooting. The repair guide covers indoor, outdoor, condensing, non-condensing and ultra-low NOx water heaters with the on-demand technology. Use the AO Smith code guide to find the main cause and solution of the following models: 110, 310, 510, 240, 340 and 540, for safety, convenience and great savings.

AO Smith tankless water heaters that are equipped with the remote controller have a great advantage; a self-diagnosing function that helps in diagnosing the heating problem. If there is a problem with the water heater, the installed remote will make a sound and the error code will display on the screen. In order to have the unit working properly, it has to be supplied with the power and correct connection.

Troubleshooting water heaters with the listed error codes below is useful for handyman homeowners, but our recommendation is always to get help from the qualified technician.

If the remote controller is not installed on the water heater, and if there is a problem, the error codes will be shown differently. For example, model 510 will show the numerical code on the 7-Seg LED on the computer board, while the 110 and 310 models only a blinking red LED that can flash one time or more. Models from the 240, 340 and 540 series will show the error codes on both temperature controller and remote controller.

It will be assumed that the tankless unit, before failure, was properly installed (also correct gas type), per manufacturer's instructions and codes. The tools needed to solve the problems from below are mainly the screwdriver, manometer and multi-meter.

AO Smith tankless error codes list

In order to fix the tankless water heater from AO Smith properly and fast, we will show all three error codes, the first for the units with the remote controller, second for the units with the 7-Seg LED and third are those heaters that do not have the remote... only blinking LED.

Error Code 03 (031), One time

Problem: Incorrect dipswitch settings.

Solution: Check for the proper settings on the computer board.

Error Code 10 (101), Five times

Problem: Warning for the "991" error code.

Solution: Check for blockage in the vents, distance between the intake and exhaust terminals are per instructions and remove if there is grease or dirt in the gas burner and fan motor.

Error Code 11 (111), Three times

Problem: The ignition failure.

Solution: Check is the Hi-limit switch properly functioning that there is no loose or broken wire connections, no soot on the flame rod or damaged computer board. Can you hear a buzzing spark ignition from the burner or two clunk sounds from the gas valve?

Error Code 12, 121, Three times

Problem: Flame loss.

Solution: Check for the proper settings on the computer board.

Error Code 31, 311, Two times

Problem: Outlet thermistor failure.

Solution: Check is there a loose or breakage in wire connection or debris on the thermistor.

Error Code 32, 321, Two times

Problem: Inlet thermistor failure.

Solution: Debris on the thermistor, loose or broken wire connection.

Error Code 331, Two times

Problem: Mixing thermistor failure.

Solution: Debris on the thermistor, loose or broken wire connection.

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Error Code 341, Two times

Problem: Exhaust thermistor failure.

Solution: See error code 331, 321 and 311.

Error Code 39, 391, Two times

Problem: Problem with the air-fuel ratio rod - AFR.

Solution: Check for the soot on the AFR rod, wire connection and breakage.

Error Code 441, Two times

Problem: Problem with the flow sensor.

Solution: heck the wire connection, wire breakage and debris on the flow sensor impeller.

Error Code 51, 55, 510, 551, Six times

Problem: Abnormal gas solenoid valve.

Solution: Check for connection and wire breakage or computer board failure (burn marks).

Error Code 61, 611, Four times

Problem: Fan motor fault.

Solution: Check for connection/breakage of wires, corrosion or frozen connectors, fan motor covered with the dust, burn marks on the computer board.

Error Code 651, Four times

Problem: Flow adjustment valve fault.

Solution: Connections or breakage of wires of the flow adjustment valve, scale buildup on the motor drive which gets locked, water leakage.

Error Code 661, Four times

Problem: Bypass valve fault (only model 540).

Solution: Connections or breakage of wires of the flow adjustment valve, scale buildup on the motor drive which gets locked, water leakage.

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Error Code 70, 701, One time

Problem: Problem with the computer board.

Solution: Check for connection and breakage of wires, burn marks on the computer board, and power supply to the unit.

Error Code 711, One time

Problem: Gas solenoid valve drive circuit failure.

Solution: See error codes 111 and 121.

Error Code 72, 721, Six times

Problem: False flame detection.

Solution: Check if the condensate drain is installed on the vent collar of the heater. Is there leaking from the heat exchanger?

Error Code 74, 741, 751

Problem: Miscommunication between the water heater and remote controller.

Solution: Check is the remote controller the right type and connections between the water heater and remote.

Error Code 761

Problem: Miscommunication in Easy-Link System.

Solution: Check the connections between water heaters.

Error Code 99, 991, Five times

Problem: Improper gas combustion.

Solution: Is the water heater set for the right elevation? Look for the blockage in the vent system and ensure if the distance between the intake and exhaust is not too small. Check is there any foreign particles or grease in the gas burner and fan motor. Is the surrounding air contaminated?

Keep in mind that the error codes are shown differently if the units are connected through the Easy-Link System... it can be seen what the error code is and which unit has it.

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