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AO Smith Tankless Error Codes and What They Mean

By Peter Gray
Updated
That flashing number is your heater pointing straight at the problem.

A freezing shower is a rough way to start the day, and an AO Smith tankless unit almost always warns you before it quits. It flashes an error code on the remote, the front display, or a blinking light.

That number is a map, not a mystery. It points to the exact part that is acting up, so you can skip the guesswork and go straight to the fix.

This guide decodes the common AO Smith tankless error codes, walks through the fixes you can safely handle yourself, and flags the faults that need a licensed pro.

Key Takeaways

  • Find the code: It shows on the remote, the digital display, or as a set number of red LED blinks you count.
  • Cut power first: Shut off both the gas and the electricity before you open the cabinet or touch anything inside.
  • Usual suspects: Most codes trace back to ignition faults, blocked venting, or a dirty sensor.
  • Reset and maintain: Most codes clear once you fix the cause, and a yearly descale prevents many of them.

In This Article

Where Can I Find the Error Code?

Troubleshooting is fairly straightforward once you locate the code, but where you look depends on your specific model. AO Smith uses three primary methods to alert you to a problem.

If you have a 240, 340, or 540 series model, check the remote control. These units display the error code directly on the remote’s screen. You will also see the code on the appliance’s front LED panel where the temperature usually sits.

For the 510 series, look at the built-in LCD display on the heater itself. Older models or specific units like the 110 and 310 series use a blinking red light system. You have to count the flashes to identify the code. For example, a light that blinks twice usually corresponds to a specific sensor error.

Check your owner’s manual to confirm exactly which interface your heater uses.

AO Smith Tankless Error Codes and Troubleshooting

Below is a list of the most common error codes for standard residential installations. We have grouped them by the number displayed on the remote, the LED display, and the corresponding blink patterns for models without screens.

For each error, we provide the following:

  • Remote: The number shown on the remote control.
  • LED: The number shown on the unit’s display.
  • Blinks: The number of times the red light flashes.

BE AWARE

Water heaters involve gas, water, and high-voltage electricity. Always turn the unit off completely (gas and electric) before performing maintenance to prevent injury (1).

Remote 03, LED 031, Light Blinks 1x

The Problem

The dip switch settings on the computer board are incorrect.

The Solution

  1. Disconnect power: Completely cut the power supply to the heater.
  2. Verify switches: Remove the front cover to locate the computer board in the center. Consult your manual and ensure the dip switches are set to the correct positions for your specific setup.

Remote 10, LED Display 101, Light Blinks 5x

The Problem

The system is detecting abnormal combustion. This generally means the ratio of air to fuel is off, often due to a lack of oxygen.

The Solution

  1. Inspect venting: Check the exhaust and intake pipes for blockages like bird nests, leaves, or ice.
  2. Check clearance: Ensure the external exhaust and intake terminals are separated by the distance specified in the installation manual to prevent exhaust recirculation.
  3. Clean internal parts: Look for dirt, dust, or grease clogging the fan motor or the gas burner mesh. Clean the intake filter if your model has one.

Remote 11, LED Display 111, Light Blinks 3x

The Problem

Ignition failure. The unit attempted to fire up three times and failed to sustain a flame.

The Solution

  1. Verify gas supply: Ensure the gas is turned on and the tank is not empty.
  2. Check the igniter: Listen for the clicking sound of the spark. If there is no spark, check the igniter wiring.
  3. Inspect the flame rod: Carbon buildup can prevent the rod from sensing the flame. Clean it gently with sandpaper or a dollar bill.
  4. Check the Hi-Limit switch: If this safety switch has tripped, the unit won’t fire. Reset it according to the manual, but call a pro if it keeps tripping.

Remote 12, LED Display 121, Light Blinks 3x

The Problem

Flame loss. The unit established a flame but lost it during operation.

The Solution

  1. Check gas pressure: Ensure your gas pressure matches the rating on the unit’s specification plate.
  2. Inspect venting: A strong backdraft or blocked vent can blow the flame out.
  3. Check electrical grounding: Improper grounding can interfere with the flame sensor reading.

Remote 31, LED Display 311, Light Blinks 2x

The Problem

The outlet thermistor (temperature sensor) is reading incorrectly or has an open/short circuit.

The Solution

  1. Check wiring: Ensure the sensor is plugged firmly into the computer board and wires are not pinched.
  2. Clean the sensor: Remove scale or debris from the sensor probe.
  3. Replace: If the wiring is good and the sensor is clean, the part likely needs replacement.

Remote 32, LED Display 321, Light Blinks 2x

The Problem

The inlet thermistor has failed.

The Solution

  1. Inspect connections: Trace the wires from the inlet sensor to the board to ensure they are secure and undamaged.
  2. Remove debris: Clean any mineral buildup off the thermistor.
  3. Swap the part: If the error persists after cleaning, install a new inlet thermistor.

Remote 33, LED Display 331, Light Blinks 2x

The Problem

The mixing thermistor has failed.

The Solution

  1. Secure connections: Check that the mixing thermistor harness is plugged in correctly.
  2. Clean: Wipe away any debris affecting the sensor reading.
  3. Replace: A persistent code usually indicates a dead sensor that requires replacement.

Remote 34, LED Display 341, Light Blinks 2x

The Problem

The exhaust thermistor has failed.

The Solution

  1. Check wiring: Verify the connection to the computer board is tight.
  2. Clean the probe: soot or condensation buildup can affect readings.
  3. Replace: Install a new exhaust thermistor if cleaning fails.

Remote 39, LED Display 391, Light Blinks 2x

The Problem

The Air-Fuel Ratio (AFR) rod is faulty or reading incorrectly.

The Solution

  1. Inspect wiring: Ensure the AFR rod connects properly to the board.
  2. Check for soot: Remove the rod and clean off any carbon or soot buildup causing false readings.

Remote 44, LED Display 441, Light Blinks 2x

The Problem

Flow sensor failure. This usually occurs in multi-unit systems when the master unit cannot activate a secondary heater.

The Solution

  1. Verify water flow: Ensure water is actually reaching the problematic unit.
  2. Inspect the filter: A clogged water inlet filter is a common cause of flow issues. Remove and rinse it.
  3. Check the impeller: Ensure the flow sensor wheel spins freely and isn’t stuck by debris.

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

The Problem

The gas solenoid valve is malfunctioning or the circuit is broken.

WARNING

If you see flames inside the unit but the hot water tap is turned off, shut down the main gas supply immediately and call a technician.

The Solution

  1. Check wiring harness: Inspect the wires connecting the gas valve assembly to the board for breaks or loose plugs.
  2. Test resistance: A technician can test the solenoid coils with a multimeter to see if the valve itself is dead.

Remote 61, LED Display 611, Light Blinks 4x

The Problem

Fan motor failure. The fan isn’t spinning at the correct speed.

The Solution

  1. Check for obstructions: Ensure nothing is physically blocking the fan blades.
  2. Look for moisture: If the computer board or fan wiring is wet, dry it out completely.
  3. Inspect the motor: If the fan spins freely by hand but not under power, the motor or the board may need replacement.

Remote 65, LED Display 651, Light Blinks 4x

The Problem

Flow adjustment valve fault. This valve regulates water volume to maintain temperature.

The Solution

  1. Inspect wiring: Look for damaged wires leading to the flow control valve.
  2. Check for scale: Hard water scale can lock the valve in place. You may need to descale the unit, circulating a descaling solution or white vinegar for at least 45 minutes and then flushing with fresh water for 10 minutes.
  3. Replace: If the valve is seized and cannot be cleaned, it requires replacement.

Remote 66, LED Display 661, Light Blinks 4x

The Problem

Bypass valve fault. This valve mixes cold water with hot to stabilize output temperature.

The Solution

  1. Check connections: Ensure the bypass valve wiring is intact.
  2. Look for leaks: Water leaking onto the valve can short the motor.
  3. Descale: Remove mineral buildup that might be jamming the valve gear.

Remote 70, LED Display 701, Light Blinks 1x

The Problem

Computer board fault. The brain of the heater is acting up.

The Solution

  1. Inspect the board: Look for visible burn marks or blown capacitors.
  2. Check connections: Ensure all plugs on the board are seated firmly.
  3. Reset power: Turn the breaker off and on to see if it was a temporary glitch. If the code returns, you need a new board.

Remote 71, LED Display 711, Light Blinks 1x

The Problem

Gas solenoid drive circuit failure. This is often related to the safety circuits on the board.

The Solution

  1. Check safety switches: Inspect the wiring on the Hi-Limit switch and overheat cutoff switch.
  2. Inspect the board: A burn mark near the gas valve connection indicates a fried board.

Remote 72, LED Display 721, Light Blinks 6x

The Problem

False flame detection. The flame rod thinks there is a fire when there shouldn’t be.

The Solution

  1. Check the condensate drain: If the drain is blocked, water can back up into the combustion chamber.
  2. Inspect for leaks: A leaking heat exchanger can cause moisture issues that confuse the sensors.
  3. Clean the flame sensor: Ensure the rod is not touching any metal parts it shouldn’t be.

Remote 74, LED Display 741 or 751

The Problem

Communication error. The remote control is not talking to the water heater correctly.

The Solution

  1. Check the wiring: Ensure the thermostat wire connecting the remote to the unit is secure at both ends.
  2. Verify the remote model: Ensure you are using a compatible remote for your specific heater.
  3. Power cycle: Turn off the power, reconnect the remote, and turn the power back on.

Remote 76, LED Display 761

The Problem

Communication error in an Easy-Link system (multiple heaters linked together).

The Solution

  1. Verify power: Ensure all linked heaters are plugged in and turned on.
  2. Check communication cables: Inspect the cables connecting the units for damage.
  3. Verify settings: Ensure the parent/child settings on the dip switches are correct according to the manual.

Remote 99, LED Display 991, Light Blinks 5x

The Problem

Imperfect combustion. The heater is struggling to burn gas efficiently, often due to airflow or pressure issues.

The Solution

  1. Check altitude settings: If you live at high elevation, ensure the dip switches are set for your altitude.
  2. Clear vents: Remove any obstructions from the exhaust or intake.
  3. Clean the system: Clean the fan motor, burner, and intake filter to ensure smooth airflow.

FAQs

How Does an AO Smith Tankless Heater Display Error Codes?

AO Smith tankless heaters show error codes in three places, depending on the model. Remote-equipped units in the 240, 340, and 540 series display the code on the remote’s LCD screen and on the front panel. The 510 series uses a built-in LCD on the heater itself. Screen-free models like the 110 and 310 series flash a red LED a set number of times, which you count against the code chart.

Why Don’t I See an Error Code on My AO Smith Water Heater?

No code with cold water usually means the unit never sensed enough flow to fire. AO Smith tankless heaters need about 0.5 GPM to start the burner and roughly 0.4 GPM to keep it running (2), so a barely-open tap or a clogged inlet filter can leave the screen quiet. If the display is completely dark instead, the heater has lost power, so check the breaker and the outlet.

Are AO Smith Water Heaters Reliable?

Yes, AO Smith tankless heaters have a strong reputation for lasting the long haul. The gas models are backed by a 15-year limited warranty on the heat exchanger and a 5-year warranty on parts (3). A yearly descale to clear mineral scale is what keeps them running reliably past the 15-year mark.

How Long Do AO Smith Tankless Water Heaters Usually Last?

An AO Smith tankless water heater lasts around 20 years with proper care, close to double the 8 to 12 years of a standard tank model. Longevity comes down to descaling: flushing the heat exchanger with vinegar or a descaling solution once a year clears the scale that would otherwise shorten its life. In hard-water areas, flush every 12 to 18 months.

What Is the Most Common Problem With AO Smith Tankless Water Heaters?

Ignition failure is the most common problem, shown as code 11 or 111. Code 111 means the burner failed to light on three straight attempts. The usual causes are a dirty flame sensor rod, low gas pressure, or a blocked intake vent, so cleaning the flame rod and confirming the gas supply clears most cases.

What Does It Mean When an AO Smith Tankless Water Heater Beeps?

Beeping signals a critical fault, most often a detected water leak. If your unit has a leak sensor, a beep means water was found inside the casing; it can also warn of overheating or a blocked exhaust. Check the display for the specific code, and shut the unit down if you see water pooling.

Why Is My AO Smith Water Heater Blinking?

A blinking red light is how a screen-free AO Smith unit reports a fault, with the number of blinks matching a code. Three blinks usually mean ignition failure, while two blinks typically point to a thermistor (sensor) problem. Count the flashes and match them to the code chart above or in your manual.

How Do You Troubleshoot an AO Smith Tankless Water Heater?

Start with the error code, then work from the simplest cause outward. Read the code on the display or remote, check the venting for blockages, confirm the gas and water valves are fully open, and clean the water inlet filter. If the code returns after a hard reset, match it to the specific part in the chart above and call a pro for gas or control-board work.

How Do I Reset My AO Smith Tankless Water Heater?

Reset an AO Smith tankless heater by power-cycling it. For a soft reset, switch off the remote or press the “Power” button on the unit, wait 30 seconds, and turn it back on. For a hard reset, flip the water heater’s breaker off, wait one minute, then flip it back on and restart. A code that returns right after a reset points to a real fault, not a glitch.

How Do I Clear the Error History?

You clear the stored error history from the computer board, not the remote. On most models you press and hold two board buttons together (often “Increase” and “Decrease”) while the unit sits in standby. The exact combination varies by model, so check your manual before you start.

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About the Author

Peter Gray

Peter writes step-by-step guides that help homeowners fix leaks, replace faucets, and handle common plumbing repairs without calling someone in. Because he worked in the trade, he can also tell you which jobs are straightforward to do yourself and which ones are worth leaving to a licensed plumber.

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Categories Water Heating