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Rheem Tankless Water Heater Error Codes

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Are you seeing an error code on your Rheem tankless water heater? Before calling a professional check and see if it's a problem you can solve yourself. Often there are a few steps you can take to troubleshoot the issue, and you just may be able to save yourself a service call!

As always, your best resource is your owners manual since it'll have the specific information you need for your model. Company's make changes through the years, and Rheem has been in business a long time with a wide range of tankless water heaters . . . there's a good chance that a change or two have taken place.

Error Codes for Rheem Tankless Water Heaters


Important: A licensed professional is your best choice to do any repairs. Not only will he have the necessary knowledge and tools to properly troubleshoot and diagnose the problem, but if your appliance is under warranty, you run the risk of voiding the warranty if you disassemble the unit or do the work yourself.

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Error Code 00

One Hour of Continuous Combustion

If you see an Error Code 00, your Rheem Tankless Water Heater has been running continuously for over an hour. Do the following to reset your system:

  • Shut off all of the hot water taps.
  • If you have a circulation pump installed, turn it off or remove it. The unit isn't designed to run continuously with a circulation system.

Error Code P1

Water Flow Too Low

An Error Code P1 indicates that the water flow is too low.

  • Check your hot water tap flow. The aerators may need to be cleaned.
  • Check and clean the water inlet filter on the tankless unit.
  • Increase the flow rate or adjust the temperature higher.
  • If the above issues are not the problem, there may be a problem with the water flow sensor.

Error Code 1L

Limescale Build-up

Maintenance is required when you see an Error Code 1L. This code indicates that your tankless needs to be flushed due to a build-up of limescale in the heat exchanger.

  • Contact a professional to flush your tankless water heater, or  perform the task yourself by following these steps.
  • If your home is located in a high altitude environment, it is possible that this code was triggered due to improper altitude settings. Contact a professional to make the proper adjustments.

Error Code 05

Imperfect Combustion Alarm

An Error Code 05 occurs when there's an issue with combustion. It could be triggered by a blockage to the Vent Exhaust or an issue with the Air Intake. In most cases, when you see this error, the unit will still be operating.

Vent Exhaust
  • Check and remove anything that may be blocking the exhaust vent.
  • Check that the vent pipes on the vent termination are connected. 
  • Double check that the venting meets the requirements (Check venting material, proper diameter and vent lengths, etc.) 
  • Check that the venting is properly sealed and that there are no obstructions.
  • You may want to contact a professional to ensure that your venting is correct and in good working order.
  • If you don't find an issue with the venting, check the blower motor.
Air Intake
  • Remove and clean the air inlet filter.
  • Clean the combustion air fan and the heat exchanger fins.
  • Check that the fan motor is operational and in good working condition. 

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Error Code 10

False Flame Detected

An Error Code 10 indicates that a false flame has been detected. Although you may want to call for service, there are a few things you can check first: 

  • Check that all vents are properly connected and that there are no obstructions and the proper materials were used during installation.
  • Check that the flue inlet and exhaust termination are not blocked and have adequate clearance space.
  • Check the heat exchanger to ensure that the fan, fins and air intake are clean and in good working order.

Error Code 11

Ignition Failure

When your Rheem tankless displays an Error Code 11 the ignition has failed. There's a number of things that could cause this error code. In most situations, its best to contact a professional to troubleshoot the problem, but there are a few things you could check:

  • First, check that you have gas to the tankless unit and that the valves are fully turned ON.
  • Check that the unit has the proper gas pressure and volume. Unless you have the tools and knowledge to work with gas appliances, you should contact a professional
  • Check that the gas solenoid valves are in good working order. They may be open or short circuited.
  • Check that the igniter is operational and in good working condition.
  • Check the igniter wiring harness for damage. 

Error Code 12

Flame Failure

Typically it's best to call a professional to troubleshoot your tankless water heater when you see an Error Code 12. But before you make the call, check the following:

  • Check that all gas valves are open and that gas is reaching the unit. 
  • Check that the flame rod wire is connected.
  • Check that the flame rod is free of carbon build-up.

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Error Code 13

Flame Rod: Poor or Improper Combustion

An Error Code 13 will only display on Indoor Rheem Tankless systems. 

  • Check that the air intake and exhaust venting is correctly installed and in good working order. It's critical that the exhaust is NOT recirculating into the unit's fresh air intake.
  • Inspect and remove any blockage from the venting and where the vent terminates outside.
  • Check and verify that the altitude settings reflect your location.
  • Check the Flame Rod FL-24.
  • Check the Blower Motor.

Error Code 14

Overheat Wrap Fault

In most cases, if the unit has unobstructed air flow and venting, it's usually best to call a professional to troubleshoot an Error Code 14.

  • Check the high fire and low fire manifold pressures.
  • Double check that the correct type of gas is being used.
  • Inspect the heat exchanger. Look for cracks and separations.
  • Check that there are no restrictions to the air flow entering the unit and no blockages at the vent terminal.
  • Check the combustion chamber and the exhaust piping for foreign materials. 

Error Code 15

Boiling Safety Device

When you see an Error Code 15 it means that the heat exchanger has maintained a temperature of 207° for over 15 seconds.

  • Check that the water heater's inlet valve is fully open.
  • Check that there are no restrictions in the cold water inlet pipe.
  • Flush the unit to remove lime scale build-up.
  • Check the heat exchanger thermistor.

Error Code 16

Over Temperature Warning

An Error Code 16 indicates that the outlet water temperature is above the remote thermostat setting.

  • Check the heat exchanger for blockages and/or limescale build-up.
  • Check that there are no airflow restrictions to the unit, and that the vent terminal is not blocked.
  • Check the outlet thermistor.
  • Check the heat exchanger thermistor.
  • Check the gas valve.

Error Code 21

Malfunction of Air Intake Filter Switch

  • Check that the air filter door is seated properly.
  • Check that the front panel is installed correctly.

Error Code 24

Switch Malfunction

If you see an error code 24, check the following:

  • Shut off the water and disconnect the remote control. Then reattempt.
  • Double check that the tankless is properly grounded.
  • Reset by pressing the MIN and MAX button on the control board (PCB).
  • Call a professional to replace the PCB.

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Error Code 29

Temperature is Too Low on the Heat Exchanger

If you receive an error code 29 on a Mid-Efficiency or Condensing unit, it indicates that the heat exchanger temperature is too low. However, it could also indicate that the neutralizer is clogged on a condensing unit. Here's where to start when you see this error code:

  • Clean the air inlet screen.
  • Clean the fins on the heat exchanger.

If you're still seeing the error code after doing the above AND your Rheem tankless is a condensing model, check the following:

  • Check that the drain line isn't blocked and that the shipping cap has been removed.
  • Inside the unit, clear all of the neutralizer drainage ports.
  • Outside the unit, clear the neutralizer drainage line.

Error Code 31

Inlet Thermistor

When your unit displays an error code 31 it indicates that there was a  fault in the water inlet temperature sensor. Depending on your comfort level, you can check the following:

  • Inspect the thermistor wiring for damage.
  • Check that the thermistor is free of limescale build-up. Clean if necessary.
  • Ohm the thermistor.

Error Code 33

Outlet Thermistor

This error code indicates that there is a fault with the outgoing water temperature sensor. Check the following before calling for service:

  • Inspect the thermistor wiring for damage.
  • Check for limescale build-up on the thermistor. Clean as needed.
  • Ohm the thermistor.
  • If the above steps check out, your sensor needs to be replaced.

Error Code 34

Ambient Thermistor

An error code 34 indicates that there is a combustion air temperature sensor fault. Check the following:

  • Inspect the thermistor wiring for damage.
  • Check and clean the ambient thermistor.
  • Ohm the thermistor. 
  • Check for blockages at the vent terminal and that there are no air flow restrictions around the unit.
  • Check the fan blade. It should fit tightly on the motor shaft and spin freely.
  • Replace the sensor. 

Error Code 35

Improper Thermistor Connection

  • Check the control board thermistor connections. All thermistors should be securely connected.

Error Code 51

Gas Control Valve

In most cases when your Rheem Tankless Water Heater displays an error code 51 your best bet is to contact a professional. Although, before you do, you can check the following:

  • Check for loose or damaged terminals on the gas control valve wiring harness.

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Error Code 52

Modulating Solenoid Valve Fault

An error code 52 is typically best handled by a professional. But you can check the following:

  • Check for loose or damaged terminals on the gas solenoid valve wiring harness.

Error Code 61

Combustion Fan Failure

If you see an error code 61, check the following:

  • Check the fan motor. It should turn freely. If there is a small amount of restriction, the motor will still operate.
  • Check for loose or damaged connections on the motor wiring harness.

Error Code 65

Water Control Valve

In most cases when you see an error code 65 its best to call a professional. However, there are a few things you can check first:

  • Check for loose or damaged connections on the water control valve wiring harness.
  • Check that the water control valve has the proper voltage.

Error Code 66

Water Bypass Valve

An error code 66 indicates a problem with the water bypass valve. Before calling for service, you can check the following:

  • Check for loose or damaged connections on the water bypass valve wiring harness.
  • Check that the water by-pass solenoid has the proper voltage.

Error Code 71

Gas Control Valve

An error code 71 indicates that there's a fault in the gas inlet solenoid valve control. This error code is best handled by a professional, but you can check the following first:

  • Check for loose or damaged connections on the gas control valve wiring harness.

Error Code 72

Flame Rod: False Flame Detected

If you see an error code 72, check the following:

  • Check that the flame rod(s) is touching the flame when the unit fires.
  • Check for foreign material inside the burner chamber. Ensure that nothing is blocking the flame at the flame rod(s)
  • Check the flame rod wiring for damage.
  • Check the flame rod for proper voltage.
  • Remove and inspect the flame rod. Clean with steel wool as needed (do not use sandpaper).

Error Code 76

Communication Fault with Remote Control

If your Rheem Tankless isn't communicating properly with the remote control, it'll throw an error code 76. Check the following before calling for service:

  • Check the wiring on the remote control. Look for damaged or loose connections.
  • Connect the remote control directly to the remote connection, which is located at the bottom of the heater. If you find that the cable is the problem, replace the cable.
  • Unplug the water heater from the electrical outlet. Then disconnect the remote control, and plug the heater back into the electrical outlet. Next, test the heater without the remote control

Error Code 79

Blower Motor Current Fault

When there's an issue with the current to the blower motor you'll see an error code 79. Your best bet is to call a professional, but if you're comfortable, you can check the following first:

  • Check that the blower motor turns freely. It'll still operate with a small amount of restriction.
  • Check that the fan motor has the proper voltage.
  • Check the fan motor for water damage that may have occurred due to condensation.
  • If the above checks out, call a professional. Either the fan motor or the PCB board needs to be replaced.

Error Code 80 & 81

Gas Control Valve

If you see either an error code 80 or 81 there's an issue with the gas cut-off valve. We highly recommend contacting a professional to troubleshoot and repair the problem. However, you can check the following:

  • Ohm out the solenoid valves.
  • Check all of the flame rods for proper voltage.
  • Replace the gas control valve.

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Error Code 82

Control Board is Not Programmed

An error code 82 may indicate that the control board is not programmed or that it doesn't have the correct gas type entered. Check the following:

  • First verify that the Program Chip is installed.
  • Then, check that the PCB is programmed for the proper type of gas (natural gas or propane).

Error Code 90

Blocked Flue/Air Intake

If you see an error code 90, check the following:

  • Check for blockages in the heat exchanger.
  • Check blockages in the blower motor.
  • Check the inlet flue and exhaust flue for blockages.

Error Code 92 & 93

Neutralizer Needs to Be Replaced

These codes will only display on condensing tankless water heaters. An error code 92 will display when the unit is  still operating  and indicates that the neutralizer should be replaced soon.

However, an error code 93 displays when the unit has  shut down  and the neutralizer must be replaced.

  • Replace the neutralizer.

Error Code 99

Blower Motor Can Not Vent

An error code 99 indicates that the appliance experienced a blocked flue fault. Check the following:

  • Check for blockages in the venting and exhaust flue. 
  • Check for blockages in the heat exchanger.
  • Check the combustion fan and inlet filter for blockages.

No Code Displays

Water Flows Through Heater But Nothing Happens

There are a number of things that could cause your Rheem Tankless to not heat water without displaying an error code. Here are a few things to check before calling for service:

  • Check the power. Is the unit plugged in? Have the circuit breakers tripped or been turned off?
  • Check the power supply inlet fuses. Are they working order and not blown?
  • If your tankless was just installed, double check that the hot and cold water lines are correctly installed and not crossed. 
  • Check that the minimum flow rate is met in order to fire the tankless.
  • Check and clean the inlet water supply filter.
  • Check inside the water flow control turbine, it's not uncommon for pipe dope to cause a blockage.
  • Test for bleed over. To do this, close the cold water supply valve to the tankless, then open the hot water faucets in the house. If you have a steady flow of water from the hot water taps, you have a bleed over problem in your system. Call a professional.

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