AO Smith Tankless Error Codes: Troubleshooting Guide

Learn about AO Smith tankless error codes for easy and inexpensive troubleshooting. The repair guide covers indoor, outdoor, condensing, non-condensing, and ultra-low NOx water heaters with on-demand technology.

Use the AO Smith code guide to find the main causes and solutions for the problems of the following models: 110, 310, 510, 240, 340, and 540 series, for safety, convenience, and great savings.


AO Smith tankless water heaters 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, an installed remote will make a sound, and the error code will display on the screen. To have the unit working properly, it has to be supplied with the power and correct connection.

A guide to troubleshooting water heaters with the listed error codes below is useful for DIY homeowners, but our recommendation is always to get help from a qualified technician.

If the remote controller is not installed on the water heater, and there is a problem, the error codes will be shown differently. For example, a model 510 will display 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.

We will assume that the tankless unit, before failure, was correctly installed (with the correct gas type), per the 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

To fix the tankless water heater from AO Smith properly and fast, we will show all three error codes (in the subtitle), the first for the units with the remote controller, the 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), Blinking One Time

Problem: Incorrect dipswitch settings.

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

Error Code 10 (101), Blinking Five Times

Problem: Warning for the “991” error code.

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

Error Code 11 (111), Blinking Three Times

Problem: The ignition failure.

Solution: Check if the Hi-limit switch properly functions and that there is no loose or broken wire connections, no soot on the flame rod, or a 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, Blinking Three Times

Problem: Flame loss.

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

Error Code 31, 311, Blinking Two Times

Problem: Outlet thermistor failure.

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

Error Code 32, 321, Blinking Two Times

Problem: Inlet thermistor failure.

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

Error Code 331, Blinking Two Times

Problem: Mixing thermistor failure.

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

Error Code 341, Blinking Two Times

Problem: Exhaust thermistor failure.

Solution: See the error codes 331, 321, and 311.

Error Code 39, 391, Blinking 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, Blinking Two Times

Problem: Problem with the flow sensor.

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

Error Code 51, 55, 510, 551, Blinking Six Times

Problem: Abnormal gas solenoid valve.

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

Error Code 61, 611, Blinking Four Times

Problem: Fan motor fault.

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

Error Code 651, Blinking Four Times

Problem: Flow adjustment valve fault.

Solution: Connections or wire breakage of the flow adjustment valve, scale buildup on the motor drive that gets locked, or there is water leakage.

Error Code 661, Blinking 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.

Error Code 70, 701, Blinking 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, Blinking One Time

Problem: Gas solenoid valve drive circuit failure.

Solution: See error codes 111 and 121.

Error Code 72, 721, Blinking 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 if the remote controller is of the right type, also connections between the water heater and remote.

Error Code 761

Problem: Miscommunication in an Easy-Link system.

Solution: Check the connections between water heaters.

Error Code 99, 991, Blinking 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 if there are any foreign particles or grease on 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.

Common Tankless Water Heater Problems

Here’s a list of common tankless water heater problems and explanations of each problem with causes, symptoms and potential solution:

  • No hot water: This can be due to a variety of reasons such as a power outage, tripped circuit breaker, malfunctioning ignition, or a faulty gas supply. Ensure the unit is powered, and the gas supply is active.
  • Fluctuating water temperatures: Sudden changes in water temperature can be caused by dirty or clogged water flow sensors, malfunctioning mixing valves, or when the demand exceeds the heater’s capacity.
  • Cold water sandwich: This refers to moments of cold water suddenly appearing between hot water intervals. It’s a result of previously heated water being flushed out, then followed by a short burst of cold water before the heater heats up the new water.
  • Mineral build-up: Hard water can lead to the accumulation of minerals like calcium and magnesium inside the heater, reducing efficiency and potentially causing blockages. Regular flushing and descaling can prevent this.
  • System overload: If too many appliances or fixtures demand hot water simultaneously, the heater might shut down due to overload. It’s essential to choose a tankless unit with an appropriate capacity for the household’s needs.
  • Venting issues: Improper installation or blockages in the vent system can cause inefficient combustion, leading to system shutdowns or the release of harmful gases.
  • Water flow issues: Low water pressure or inconsistent flow can result from clogged filters, blocked vents, or malfunctioning flow sensors.
  • Ignition or flame failures: This can be caused by gas supply issues, faulty igniters, dirty or misaligned flame rods, or problems with the unit’s electronic board.
  • Condensation issues: Some condensation is normal, but excessive condensation can indicate a malfunctioning flue or improper combustion, leading to potential water damage.

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