AO Smith Tankless Error Code Troubleshooting

Updated
Categories Water Heating
Has your AO tankless water heater gone on the blink? Learn what the error codes mean and how to troubleshoot them.

What happens when you turn on the hot water faucet and all you get is cold water? Maybe you go to take a hot shower and find your water heater has failed. Not having instant access is frustrating, to say the least!

And it doesn’t help to find a mysterious water heater code flashing at you either. Don’t worry — we’re here to help with tips on understanding those AO Smith tankless error codes.

Time to look at these error codes, what they mean, and steps you can take to troubleshoot them. Some you can fix yourself, but others need a qualified technician.

Table of Contents

    Where Can I Find the Error Code?

    With this brand of tankless water heater, the display of the error codes is different from model to model.

    Some models, like the 240, 340 and 540 series, have a remote control which will display an error code and alert you that something is wrong. These models will also display the error code on the LED display on the appliance, where you’d normally see the temperature.

    The 510 series will display the error code on the LCD display. Other models, like the 110 and 310, will show a flashing red light on the appliance. It blinks for a different number of times depending on the error.

    You can check your owner’s manual to see what model you have and where to find the error code.

    AO Smith Tankless Error Codes and Troubleshooting

    Assuming that your water heater has been installed correctly according to the manufacturer’s specifications, these are the error codes you will see.

    For each error, we indicate:

    • The numbers for the remote control
    • The LED display numbers
    • Amount of times the light will blink

    BE AWARE

    Water heaters are powered by gas and electric. Please turn them off before doing any maintenance so you don’t harm yourself (1).

    Remote 03, LED 031, Light Blinks 1x

    What It’s Telling You

    • The dip switch settings aren’t correct.

    The Solution

    1. Disconnect the power: Turn off the power supply.
    2. Check the dip switches: Remove the front cover and find the dip switches. They’ll be at the center of the computer board. Make sure the settings are correct, according to your users manual.

    Remote 10, LED Display 101, Light Blinks 5x

    What It’s Telling You

    • This is a warning code for abnormal combustion.

    The Solution

    1. Air flow: Check the venting for obstructions like bird nests, leaves, etc.
    2. Check clearance: Exhaust and intake terminals need to have the correct clearance between them.
    3. Clean: Check for dirt or grease in the fan motor and gas burner, and clean if necessary. Make sure the intake is free from dust and grease.

    Remote 11, LED Display 111, Light Blinks 3x

    What It’s Telling You

    • There is an ignition failure where the unit doesn’t fire up after three failed attempts.

    The Solution

    1. Check connections: Make sure the hi-limit switch is working properly and that there are no loose wires or connections. If it has tripped, turn the power off and reset it but call out service to find out why it’s tripping.
    2. Flame rod: Make sure this is free from soot.
    3. Computer board: Check that the computer board isn’t damaged and there are no loose wires or connections.

    Remote 12, LED Display 121, Light Blinks 3x

    What It’s Telling You

    • There is a loss of flame during operation.

    The Solution

    1. Tripped switch: Make sure the hi-limit switch hasn’t tripped.
    2. Computer board: Verify the computer board settings are correct, in line with your owners manual.
    3. Obstructions: Check there’s no blockage in the gas supply, and that the gas is on and at the right pressure. Look for blockages in the vents.

    Remote 31, LED Display 311, Light Blinks 2x

    What It’s Telling You

    • There’s a failure in the outlet thermistor.

    The Solution

    1. Connections: Ensure the thermistor wiring connection is good and there’s no damage.
    2. Clean: Check the thermistor for dirt or debris and clean if necessary.
    3. Change out parts: If the code error persists, replace the outlet thermistor.

    Remote 32, LED Display 321, Light Blinks 2x

    What It’s Telling You

    • There’s a failure in the inlet thermistor.

    The Solution

    1. Connections: Inspect the thermistor wiring, make sure the connection is good and there’s no damage.
    2. Clean: Check the thermistor for dirt or debris and clean if necessary.
    3. Change out parts: If the code error persists, replace the inlet thermistor.

    Error Code: 331, Light Blinks 2x

    What It’s Telling You

    • There’s a failure in the mixing thermistor.

    The Solution

    1. Connections: Check the thermistor wiring, make sure the connection is good and there’s no damage.
    2. Clean: Check the thermistor for dirt or debris and clean if necessary.
    3. Change out parts: If the code error persists, replace the mixing thermistor.

    Error Code: 341, Light Blinks 2x

    What It’s Telling You

    • The exhaust thermistor has failed.

    The Solution

    1. Connections: Check the thermistor wiring, make sure the connection is good and there’s no damage.
    2. Clean: Examine the thermistor for dirt or debris and clean if necessary.
    3. Change out parts: If the code error persists, change replace the exhaust thermistor.

    Remote 39, LED Display 391, Light Blinks 2x

    What It’s Telling You

    • The air-fuel ratio rod (AFR) is faulty.

    The Solution

    1. Connections: Make sure the wiring is connected properly and isn’t damaged.
    2. Clean: Check the AFR rod for soot and clean if necessary.

    Error Code: 441, Light Blinks 2x

    What It’s Telling You

    • There’s an issue with the flow sensor. This happens when you have more than one heater connected and the master unit signals another heater to start and it fails.

    The Solution

    1. Connections: Verify the heater that has failed has a water supply. Ensure the wires to the computer board aren’t damaged.
    2. Air flow: Make sure the inlet filter is free of debris
    3. Clean: Check that the flow sensor has no dirt on it.

    Remote 51, 55, LED Display 510, 511, Light Blinks 6x

    What It’s Telling You

    • There’s a problem with the gas solenoid valve.

    WARNING

    If the hot water is off but you can still see flames in the appliance, turn off the gas and power immediately. Call a service technician.

    The Solution

    1. Check for damage: Make sure the wires to the gas valve assembly aren’t damaged.
    2. Test pressure levels: Ensure the gas pressure is at the right level.

    Remote 61, LED Display 611, Light Blinks 4x

    What It’s Telling You

    • The fan motor is faulty.

    The Solution

    1. Damage: Check the wiring to the fan motor for damage and the PC board for burn marks or moisture. If there is moisture, let it dry completely before turning the power back on.
    2. Clean: Make sure there’s no water or dust in the fan housing or on the motor. If there is, dry or clean it.
    3. Change parts: If the error persists, the computer board might need changing.

    Error Code: 651, Light Blinks 4x

    What It’s Telling You

    • There’s a faulty flow adjustment valve.

    The Solution

    1. Damage: Check the wiring to the flow adjustment valve for damage or breaks.
    2. Inspect for leaks: Make sure the valve isn’t leaking.
    3. Clean: See if there’s any build-up on the valve and clean it.
    4. Replace parts: If it can’t be cleaned, change out the valve.

    Error Code: 661, Light Blinks 4x

    What It’s Telling You

    • There’s a fault with the bypass valve.

    The Solution

    1. Damage: Check the wiring to the flow adjustment valve for damage.
    2. Clean: Make sure there’s no scale build-up on the motor drive.
    3. Leaks: Check for water leakage from the valve.

    Remote 70, LED Display 701, Light Blinks 1x

    What It’s Telling You

    • The computer board is experiencing a problem.

    The Solution

    1. Connections: Check there are no breaks in the wires and that they are connected properly.
    2. Damage: Inspect the board closely. Can you see burn marks?
    3. Power: Make sure there is a power supply to the unit.

    Error Code: 711, Light Blinks 1x

    What It’s Telling You

    • There’s a failure in the gas solenoid valve drive circuit, hi-limit valve or overheat cut off switch. This code might appear in place of codes 111 or 121.

    The Solution

    1. Damage: Check the hi-limit switch and overheat cut-off switch for wire damage.
    2. Connections: Inspect the control board connection for the gas valve.

    Remote 72, LED Display 721, Light Blinks 6x

    What It’s Telling You

    • There’s a false flame detection.

    The Solution

    1. Power: Firstly, turn off the power.
    2. Check the condensate drain: Make sure it’s installed on the heaters vent collar.
    3. Leaks: Inspect the appliance for leakage from the heat exchanger.
    4. If in doubt: The issue could be the flame sensor, but just in case it isn’t, you should call a service technician.

    Remote 74, LED Display 741 and 751

    What It’s Telling You

    • There’s miscommunication between the remote controller and the water heater. This can happen if you disconnect the remote from the heater while the power supply is on.

    The Solution

    1. Power: Turn the power off before disconnecting the remote.
    2. Connections: Check the connections between the remote and the water heater. Make sure you have the correct remote controller.
    3. Check for a power surge: Look for signs like burnt out wires or damaged control board. A minor surge or outage might cause LCD displays to flash where power has been lost.

    Error Code: LED Display 761

    What It’s Telling You

    • There’s a miscommunication between heaters in an Easy-Link system. The error code will appear on the master heater and will indicate which heater is at fault.

    The Solution

    1. Check the power: Make sure the heater that has lost communication has power.
    2. Make sure the units are “talking” to each other: Check that the communication system is correctly set up in line with your user manual.

    Remote 99, LED Display 991, Light Blinks 5x

    What It’s Telling You

    • There’s improper gas combustion.

    The Solution

    1. Make sure the altitude setting is correct: Check the water heater is set for the right elevation.
    2. Blockages: Ensure there are no obstructions in the venting system.
    3. Check the clearance: Verify the intake and exhaust are the correct distance apart.
    4. Clean: Look for grease, dirt or debris on the gas burner or fan motor and remove them.

    May the Flow Be with You…

    Now you know a little more about AO Smith tankless error codes, you can start fixing any problems yourself. Just remember, call a service technician if you aren’t sure about anything. When gas and electrics are involved, safety is paramount.

    We hope you found our guide useful and informative. Please leave us a comment with any tips or questions, and don’t forget to share with another AO Smith owner!

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    About the Author

    Peter Gray

    Peter has been a homeowner for 35+ years and has always done his own repair and improvement tasks. As a retired plumber, Peter now spends his time teaching others how they can fix leaks, replace faucets, and make home improvements on a budget.
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