A. O. Smith tank water heaters flash a status light on the gas control a set number of times, and tankless and hybrid models show a code on the display. Below are 26 A. O. Smith codes across 4 model series. Each one says what it means, which series it applies to, what you can safely check, and when to call a licensed pro, with a link to the A. O. Smith document it comes from.
The same code can mean different things on different series, so match the series column to your unit. Your unit's manual is the final word. Reviewed October 2026.
31 codes
| Code | Meaning | Applies to | Safety |
|---|---|---|---|
| 03 110/310: red LED on the computer board blinks one time. Remote controller shows 03. 510 (T-D2): 7-segment display shows 031 | Incorrect DIP switch setting | A. O. Smith 110 (T-KJr2), 310 (T-K4) and 510 (T-D2) tankless water heaters | 🛑 Call a pro |
| 10 110/310: red LED on the computer board blinks five times. Remote controller shows 10. 510 (T-D2): 7-segment display shows 101 | Warning for the 991 code (air or exhaust problem) | A. O. Smith 110 (T-KJr2), 310 (T-K4) and 510 (T-D2) tankless water heaters | 🧭 Check first |
| 10 Error code flashes on the temperature controller display | Air supply or exhaust blockage | A. O. Smith 305, 505 and 705 direct vent indoor tankless water heaters | 🧭 Check first |
| 11 110/310: red LED on the computer board blinks three times. Remote controller shows 11. 510 (T-D2): 7-segment display shows 111 | Ignition failure | A. O. Smith 110 (T-KJr2), 310 (T-K4) and 510 (T-D2) tankless water heaters | 🛑 Call a pro |
| 11 Error code flashes on the temperature controller display | No ignition | A. O. Smith 305, 505 and 705 direct vent indoor tankless water heaters | 🛑 Call a pro |
| 12 110/310: red LED on the computer board blinks three times. Remote controller shows 12. 510 (T-D2): 7-segment display shows 121 | Loss of flame | A. O. Smith 110 (T-KJr2), 310 (T-K4) and 510 (T-D2) tankless water heaters | 🛑 Call a pro |
| 12 Error code flashes on the temperature controller display | Flame failure | A. O. Smith 305, 505 and 705 direct vent indoor tankless water heaters | 🛑 Call a pro |
| 31 110/310: red LED on the computer board blinks two times. Remote controller shows 31. 510 (T-D2): 7-segment display shows 311 | Outlet thermistor failure | A. O. Smith 110 (T-KJr2), 310 (T-K4) and 510 (T-D2) tankless water heaters | 🛑 Call a pro |
| 32 110/310: red LED on the computer board blinks two times. Remote controller shows 32. 510 (T-D2): 7-segment display shows 321 | Inlet thermistor failure | A. O. Smith 110 (T-KJr2), 310 (T-K4) and 510 (T-D2) tankless water heaters | 🛑 Call a pro |
| 61 110/310: red LED on the computer board blinks four times. Remote controller shows 61. 510 (T-D2): 7-segment display shows 611 | Fan motor fault | A. O. Smith 110 (T-KJr2), 310 (T-K4) and 510 (T-D2) tankless water heaters | 🛑 Call a pro |
| 61 Error code flashes on the temperature controller display | Combustion fan failure | A. O. Smith 305, 505 and 705 direct vent indoor tankless water heaters | 🛑 Call a pro |
| 72 110/310: red LED on the computer board blinks six times. Remote controller shows 72. 510 (T-D2): 7-segment display shows 721 | False flame detection | A. O. Smith 110 (T-KJr2), 310 (T-K4) and 510 (T-D2) tankless water heaters | 🛑 Call a pro |
| 72 Error code flashes on the temperature controller display | Flame sensing device fault | A. O. Smith 305, 505 and 705 direct vent indoor tankless water heaters | 🛑 Call a pro |
| 74 110/310: no blink pattern listed (N/A). Remote controller shows 74. 510 (T-D2): remote shows 741 | Remote controller communication error | A. O. Smith 110 (T-KJr2), 310 (T-K4) and 510 (T-D2) tankless water heaters | 🧭 Check first |
| 99 110/310: red LED on the computer board blinks five times. Remote controller shows 99. 510 (T-D2): 7-segment display shows 991 | Imperfect combustion | A. O. Smith 110 (T-KJr2), 310 (T-K4) and 510 (T-D2) tankless water heaters | 🧭 Check first |
| 02 Error code flashes on the temperature controller display | No burner operation in freeze protection mode | A. O. Smith 305, 505 and 705 direct vent indoor tankless water heaters | 🛑 Call a pro |
| 14 Error code flashes on the temperature controller display | Thermal fuse | A. O. Smith 305, 505 and 705 direct vent indoor tankless water heaters | 🛑 Call a pro |
| 16 Error code flashes on the temperature controller display | Over temperature warning | A. O. Smith 305, 505 and 705 direct vent indoor tankless water heaters | 🧭 Check first |
| LC Error code flashes on the temperature controller display | Scale build-up in heat exchanger | A. O. Smith 305, 505 and 705 direct vent indoor tankless water heaters | 🧭 Check first |
| 2 flashes Gas control valve status light flashes, then a pause | Thermopile voltage low | A. O. Smith Direct Vent Residential Gas tank water heaters (Ultra Low NOx models) with gas control valve/thermostat status light | 🛑 Call a pro |
| 4 flashes Gas control valve status light flashes, then a pause | Overheat failure | A. O. Smith Direct Vent Residential Gas tank water heaters (Ultra Low NOx models) with gas control valve/thermostat status light | 🛑 Call a pro |
| 5 flashes Gas control valve status light flashes, then a pause | Sensor failure | A. O. Smith Direct Vent Residential Gas tank water heaters (Ultra Low NOx models) with gas control valve/thermostat status light | 🛑 Call a pro |
| 7 flashes Gas control valve status light flashes, then a pause | Electronic control failure | A. O. Smith Direct Vent Residential Gas tank water heaters (Ultra Low NOx models) with gas control valve/thermostat status light | 🛑 Call a pro |
| 8 flashes Gas control valve status light flashes, then a pause | Pilot or thermopile still active with the knob off | A. O. Smith Direct Vent Residential Gas tank water heaters (Ultra Low NOx models) with gas control valve/thermostat status light | 🛑 Call a pro |
| SF Shown on the water heater display | Air filter is dirty | A. O. Smith Hybrid Electric Heat Pump water heaters (Voltex), Installation Instructions and Use & Care Guide 100269085 | 🧭 Check first |
| ECF Shown on the water heater display | Heat pump compressor starting and stopping often | A. O. Smith Hybrid Electric Heat Pump water heaters (Voltex), Installation Instructions and Use & Care Guide 100269085 | 🧭 Check first |
| Edr Shown on the water heater display | Not enough water in tank (dry fire) | A. O. Smith Hybrid Electric Heat Pump water heaters (Voltex), Installation Instructions and Use & Care Guide 100269085 | 🧭 Check first |
| EPL Shown on the water heater display | Power supply voltage too low | A. O. Smith Hybrid Electric Heat Pump water heaters (Voltex), Installation Instructions and Use & Care Guide 100269085 | 🧭 Check first |
| EUC Shown on the water heater display | Upper element not functioning | A. O. Smith Hybrid Electric Heat Pump water heaters (Voltex), Installation Instructions and Use & Care Guide 100269085 | 🛑 Call a pro |
| ELC Shown on the water heater display | Lower element not functioning | A. O. Smith Hybrid Electric Heat Pump water heaters (Voltex), Installation Instructions and Use & Care Guide 100269085 | 🛑 Call a pro |
| ICE Shown on the water heater display | Heat pump is defrosting | A. O. Smith Hybrid Electric Heat Pump water heaters (Voltex), Installation Instructions and Use & Care Guide 100269085 | ✅ Keep running |
✅ Usually fine to keep running · 🧭 Check a few things first · 🛑 Turn it off and call a licensed pro
The codes A. O. Smith documents for each series we have checked. If your series is not listed, the code may mean something different on your unit, so check its manual.
Applies to: A. O. Smith 110 (T-KJr2), 310 (T-K4) and 510 (T-D2) tankless water heaters. Shows as: 110/310: red LED on the computer board blinks one time. Remote controller shows 03. 510 (T-D2): 7-segment display shows 031
The manual lists this as: Incorrect DIPswitch setting.
When to call a pro: The DIP switches are on the circuit board. Have a qualified installer or service technician check the settings.
Applies to: A. O. Smith 110 (T-KJr2), 310 (T-K4) and 510 (T-D2) tankless water heaters. Shows as: 110/310: red LED on the computer board blinks two times. Remote controller shows 31. 510 (T-D2): 7-segment display shows 311
The manual lists this as: Outlet thermistor failure.
When to call a pro: Call a licensed service pro. The manual says to consult the manufacturer after the listed checks, and these are gas, electrical or board parts.
Applies to: A. O. Smith 110 (T-KJr2), 310 (T-K4) and 510 (T-D2) tankless water heaters. Shows as: 110/310: red LED on the computer board blinks two times. Remote controller shows 32. 510 (T-D2): 7-segment display shows 321
The manual lists this as: Inlet thermistor failure.
When to call a pro: Call a licensed service pro. The manual says to consult the manufacturer after the listed checks, and these are gas, electrical or board parts.
Applies to: A. O. Smith 110 (T-KJr2), 310 (T-K4) and 510 (T-D2) tankless water heaters. Shows as: 110/310: red LED on the computer board blinks four times. Remote controller shows 61. 510 (T-D2): 7-segment display shows 611
The manual lists this as: Fan motor fault.
When to call a pro: Call a licensed service pro. The manual says to consult the manufacturer after the listed checks, and these are gas, electrical or board parts.
Applies to: A. O. Smith 305, 505 and 705 direct vent indoor tankless water heaters. Shows as: Error code flashes on the temperature controller display
The manual lists this as combustion fan failure.
When to call a pro: Call a licensed pro. The manual warns that some remedies involve gas or electricity and need a qualified service technician.
Applies to: A. O. Smith 110 (T-KJr2), 310 (T-K4) and 510 (T-D2) tankless water heaters. Shows as: 110/310: red LED on the computer board blinks six times. Remote controller shows 72. 510 (T-D2): 7-segment display shows 721
The manual lists this as: False flame detection.
When to call a pro: Call a licensed pro. The manual says to check the condensate drain on the vent collar and for heat exchanger leaks.
Applies to: A. O. Smith 305, 505 and 705 direct vent indoor tankless water heaters. Shows as: Error code flashes on the temperature controller display
The manual lists this as a flame sensing device (flame rod) fault.
When to call a pro: Call a licensed pro. The manual warns that some remedies involve gas or electricity and need a qualified service technician.
Applies to: A. O. Smith 110 (T-KJr2), 310 (T-K4) and 510 (T-D2) tankless water heaters. Shows as: 110/310: no blink pattern listed (N/A). Remote controller shows 74. 510 (T-D2): remote shows 741
The manual lists this as: Miscommunication between water heater and remote controller.
When to call a pro: If the remote still cannot communicate, call a qualified service technician to check the controller model and connections.
Applies to: A. O. Smith 305, 505 and 705 direct vent indoor tankless water heaters. Shows as: Error code flashes on the temperature controller display
The manual lists this as no burner operation during freeze protection mode, with the remedy "Service Call".
When to call a pro: The manual's remedy is a service call.
Applies to: A. O. Smith 305, 505 and 705 direct vent indoor tankless water heaters. Shows as: Error code flashes on the temperature controller display
The manual lists this as a thermal fuse fault.
When to call a pro: Call a licensed pro. The manual warns that some remedies involve gas or electricity and need a qualified service technician.
Applies to: A. O. Smith 305, 505 and 705 direct vent indoor tankless water heaters. Shows as: Error code flashes on the temperature controller display
The manual lists this as an over temperature warning.
When to call a pro: Call a licensed pro if the warning returns. The manual lists low circulating flow, debris and a clogged heat exchanger as causes to check.
Applies to: A. O. Smith Direct Vent Residential Gas tank water heaters (Ultra Low NOx models) with gas control valve/thermostat status light. Shows as: Gas control valve status light flashes, then a pause
Five flashes indicate a sensor failure in the gas control valve/thermostat.
When to call a pro: The chart says to replace the gas control valve/thermostat. Call a licensed pro.
Applies to: A. O. Smith Direct Vent Residential Gas tank water heaters (Ultra Low NOx models) with gas control valve/thermostat status light. Shows as: Gas control valve status light flashes, then a pause
Seven flashes indicate a failure of the electronic control in the gas control valve/thermostat.
When to call a pro: The chart says to replace the gas control valve/thermostat. Call a licensed pro.
Applies to: A. O. Smith Direct Vent Residential Gas tank water heaters (Ultra Low NOx models) with gas control valve/thermostat status light. Shows as: Gas control valve status light flashes, then a pause
Eight flashes appear only if the temperature knob has been turned off and the thermopile still produces power, for example because the pilot valve is stuck open.
When to call a pro: The chart says to replace the gas control valve/thermostat if the pilot stays lit with the knob off. Call a licensed pro.
Applies to: A. O. Smith Hybrid Electric Heat Pump water heaters (Voltex), Installation Instructions and Use & Care Guide 100269085. Shows as: Shown on the water heater display
The guide says the heat pump compressor is starting and stopping frequently.
When to call a pro: The guide says that if the error persists, contact a qualified person to check the fan wire connection.
Applies to: A. O. Smith Hybrid Electric Heat Pump water heaters (Voltex), Installation Instructions and Use & Care Guide 100269085. Shows as: Shown on the water heater display
The guide says the power supply voltage is too low.
When to call a pro: Call a licensed electrician or qualified person to check the supply voltage.
Applies to: A. O. Smith Hybrid Electric Heat Pump water heaters (Voltex), Installation Instructions and Use & Care Guide 100269085. Shows as: Shown on the water heater display
The guide says the upper heating element is not functioning.
When to call a pro: The guide says to check for a loose connection at the element and replace a failed element. Call a qualified person.
Applies to: A. O. Smith Hybrid Electric Heat Pump water heaters (Voltex), Installation Instructions and Use & Care Guide 100269085. Shows as: Shown on the water heater display
The guide says the lower heating element is not functioning.
When to call a pro: The guide says to check for a loose connection at the element and replace a failed element. Call a qualified person.
Applies to: A. O. Smith Hybrid Electric Heat Pump water heaters (Voltex), Installation Instructions and Use & Care Guide 100269085. Shows as: Shown on the water heater display
The guide says this means the heat pump is in a defrosting cycle. It is normal operation.
When to call a pro: Call a qualified person only if the unit never leaves this state.
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