Troubleshooting Tankless Water Heater Error Codes: Rheem


Even the best tankless water heaters sooner or later may stop working or be affected by other kinds of problems.

So is the case with Rheem tankless water heaters. The good news here is that Rheem units have a set of error codes displayed when something out of the ordinary happens.

Water heater beeping? See our article Why is My Tankless Water Heater Beeping?

When that happens, all you need to do is know what each error code means so that you can take the appropriate steps in rectifying the situation.

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Error Code 00: One Hour of Continuous Combustion

Rheem error code 00 indicates that the unit has been working continuously for more than one hour. If your Rheem tankless water heater is showing an error code 00, it must be restarted. Close the hot water taps and remove or turn off the circulation pump if there’s any installed.

Error Code 03: Manifold Control Failure

Rheem error code 03 means there is a problem with the manifold control. Rheem error code 03 is found on units with MIC-180 or EZLink. Ensure the MIC-180 and the EZLink are installed and set up correctly. Replace the PCB or the communications cable if necessary.

Error Code 05: Imperfect Combustion Alarm

Rheem error code 05 means there is a problem with the combustion. Rheem error code 05 can be caused by blocked air ventilation and exhaust systems. Inspecting their condition and leaning them will normally fix this error code. If the error code persists, contact a licensed technician.

Check the air inlet filter and clean it and ensure the vent pipes are all connected correctly.

Error Code 10: False Flame Detected

Rheem error code 10 means a false flame has been detected. To troubleshoot the Rheem error code 10, check the condition of the air intake, exhaust inlet, heat exchanger, fan, and fins. Ensure all vents are not blocked and in good working condition. If the code persists, call a licensed contractor.

Error Code 11: No ignition

Rheem error code 11 indicates that the ignition has failed. Rheem error code 11 can be caused by low gas pressure, faulty or improperly wired gas solenoid valve, and igniter. If your unit displays an error code 11, it is advisable to contact a licensed professional.

Error Code 12: Flame Failure

Rheem error code 12 indicates a flame failure. If your Rheem displays an error code 12, ensure all gas valves are open and adequate gas pressure. Ensure the flame rod is wired correctly and clean. If the error code 12 persists, contact a licensed technician.

Error Code 13: Flame Rod: Poor or Improper Combustion

Rheem error code 13 indicates a problem with the flame rod, which is typically accompanied by poor or improper combustion and found on indoor units. Inspect the condition of the flame rod and blower motor, the air intake, and exhaust vents. Ensure all parts are in good working condition, clean, and not blocked by a foreign object.

Ensure that the altitude is set correctly for your particular needs and the vents meet the necessary size requirements for safe operation and use. More information can be found in your owner’s manual.

Error Code 14: Overheat Film Wrap

Rheem error code 14 means the overheat film wrap has been activated, usually caused by a hot spot. The Rheem error code 14 can be caused by using the wrong type of gas, blocked or obstructed airflow, vent terminals and exhaust vents, and faulty or damaged heat exchanger.

Ensure there are no build-ups, foreign objects, or deposits of any sort near or around the unit and its venting system and combustion chamber.

The Molex connections’ resistance should also be measured. Before doing so, remember to remove the Molex from the CB. Ensure it is installed correctly and measure its resistance. If the resistance is not according to specifications, it should be replaced. However, if the resistance seems good, the PCB may need to be replaced.

Error Code 15: Boiling Safety Device

Rheem error code 15 means the heat exchanger is overheating. Check the vent terminals and exhaust venting and ensure nothing blocks the airflow near or around the unit. Check the heat exchanger and flush the unit if there are scale build-ups. The heat exchanger thermistor or the PCB may need to be replaced.

Rheem error code 15 is displayed when the heat exchanger reaches and maintains 207°F for 15 seconds, and at 210°F, it will automatically shut down and switch on automatically when the temperature goes down to 194°F. If the unit reaches 230°F, however, it will again display an error code 15, shut down, but not start automatically on its own.

As part of the troubleshooting process, check the heat exchanger’s resistance reading. If the readings are not within specifications, you may be dealing with a faulty thermistor, which should be replaced. If the readings are looking good, the PCB may be at fault.

Error Code 16: Over Temperature Warning

Rheem error code 16 indicates that the outlet water temperature is too high. Ensure the airflow is not restricted or blocked. Check the vent terminal and exhaust venting. Check the heat exchanger for scale build-up. Inspect the condition of the outlet thermistor, heat exchanger thermistors, and the gas valve.

If the water’s temperature is too high, this should be indicated in the maintenance information. Check the water flow servo and the cold water bypass system. Ensure they are in good working condition, and there is no blockage along the piping.

If the maintenance information does not indicate the water being too hot, check the hot outlet thermistors resistance readings. If the readings are not right, replace the thermistor, and if they are within specs, you may have to replace the PCB.

Error Code 21: Malfunction of Air Intake Filter Switch

Rheem error code 21 means the air intake filter is not working correctly. Rheem error code is found on indoor units only and can be caused by a dirty air filter door or improperly installed and seated air filter door or front panel. You may need to replace the air filter switch assy or the PCB.

After removing the front panel cover, press the air intake switch and ensure it operates correctly. Measure its resistance and voltage and replace it if necessary. If all seems good, the problem may be due to a faulty PCB.

Error Code 24: Switch Malfunction

Rheem error code 24 indicates a problem with an operational switch. To troubleshoot Rheem error code 24, switch off the unit and check if it works with the remote control disconnected. Ensure the unit is properly grounded and its wiring is in good condition. Reset the unit using the Min/Max switches on the PCB.

While troubleshooting your Rheem, do not operate it and do not open the hot water tap, especially while testing it using the Min and Max buttons.

If the unit does not work after resetting it using the Min and Max buttons, you may be dealing with a faulty PCB.

Error Code 29: Temperature is Too Low on the Heat Exchanger

Rheem error code 29 indicates that the outlet temperature is too low. If your Rheem displays an error code 29, ensure the heat exchanger is clean of any build-ups. Clean the inlet filter if necessary. You may need to replace the heat exchanger thermistor or PCB.

If the water temperature is displayed in the maintenance information, the unit should be working properly. However, if it isn’t, check the resistance readings of the heat exchanger. Replace it if necessary. If the readings are within specs, you may have to replace the PCB.

On some Rheem units, error code 29 can indicate a clogged neutralizer. Rheem error code can display when the neutralizer or drain pipe gets clogged. Remove and clean any build-ups and blockage in the condensate drain system. Ensure the condensate line is installed correctly and is within specifications.

During the winter, water can freeze in the drain piping, effectively blocking it and causing an error code 29 to appear on the unit.

Error Code 31: Inlet Thermistor

Rheem error code 31 indicates a malfunction with the water inlet thermistor. The Rheem error code 31 can be caused by scale build-up on the inlet water filter or any thermistors, damaged or bad wiring, and short circuits. You may need to replace the thermistor or the PCB.

During troubleshooting the error code 31, check what the water temperature according to the maintenance information is—a temperature of 32°F indicates a thermistor that is not making a good connection, and 199°F indicates a direct short.

If there is no information, check the inlet thermistor’s resistance readings and replace the thermistor if they are not good. If the readings are okay, you may be dealing with a faulty PCB.

Error Code 32:

Rheem error code 32 means there is a problem with the heat exchanger thermistor. Rheem error code 32 can be caused by scale build-up on the inlet water filter or any other thermistors, damaged or bad wiring, and short circuits. You may need to replace the thermistor or the PCB.

Check if the maintenance information displays the heat exchanger’s temperature.

If it does, the unit should be working properly and restarting. 

If there is no information, however, measure the resistance to the heat exchanger thermistor. If the temperature is not within specifications, replace the thermistor, and if it is, you may have to replace the PCB.

A temperature of 32°F means the thermistor is not connected correctly, and 199°F indicates a direct short.

Error Code 33: Outlet Thermistor

Rheem error code 33 indicates a problem with the hot water outlet temperature thermistor. Rheem error code 33 can be caused by scale build-up on the inlet water filter or any other thermistors, damaged or bad wiring, and short circuits. You may need to replace the thermistor or the PCB.

Check if the maintenance information displays the hot water outlet temperatures. If it does, the unit should be good and restarted. However, if it doesn’t, the thermistor’s resistance should be measured, and the thermistor or the PCB may need to be replaced.

A temperature of 32°F indicates a thermistor that does not make a good connection, and 199°F indicates a direct short.

Error Code 34: Ambient Thermistor

Rheem error code 34 indicates a problem with the ambient combustion air thermistor. Rheem error code 34 can be caused by scale build-up on the ambient combustion air sensor, blocked vent terminals, fan blades, airflow, damaged or bad wiring, and short circuits. You may need to replace the thermistor or the PCB.

Check if the maintenance information displays the ambient air temperature. 

If there is information, the unit should be working properly and simply require restarting. 

If there is no information, the sensor’s resistance should be measured, and the sensor should be replaced if not within specifications. The PCB may need to be replaced if the sensor is working properly.

A temperature of 32°F means the thermistor that does not make a good connection, and 199°F means a direct short.

Error Code 35: Improper Thermistor Connection

Rheem error code 35 indicates there is a problem with the connections of one or several thermistors. If your Rheem displays an error code 35, inspect and ensure all thermistor(s), connections are not loose and installed correctly. Check the wiring, the Molex connections, and ensure there are no scale build-ups or debris.

Make sure to check all four sensors. These are:

  • Cold water inlet sensor.
  • Hot water outlet sensor.
  • Heat exchanger sensor.
  • Ambient air sensor.

Check the resistance readings of each sensor and replace the sensors if not working correctly.

If everything looks in good condition, the PCB may have to be replaced.

Error Code 38: Oxygen Depletion Sensor

Rheem error code 38 indicates a problem with the oxygen depletion sensor. If your Rheem displays error code 38, light the burner for a few minutes and measure the sensor’s current readings. If there is enough current running, you may need to replace the burner assembly; otherwise, replace the PCB.

Error Code 51: Gas Control Valve

Rheem error code 51 indicates a problem with the gas inlet solenoid valve. If your Rheem displays an error code 51, ensure the gas valve wiring is in good working condition and installed correctly. The gas control assembly, the main burner, the flame rod electrodes, or the PCB may need to be replaced.

Troubleshooting of error code 51 involves turning the unit off and on two times at 10-second intervals and inspecting the view window for any signs of a remaining flame.

If there is a remaining flame, the gas control assembly should be replaced.

If there is no remaining flame, the display maintenance should be checked for information regarding the flame rod. If there is no information, the unit should be working correctly. However, if there is information and depending on its type, the main burner and flame rod electrodes or the PCB may need to be replaced.

Check your unit’s troubleshooting manual for more, model-specific, information.

Error Code 52: Modulating Solenoid Valve Fault

Rheem error code 52 indicates an issue with the proportional gas flow regulator valve (PGFR). Rheem error code 52 can be caused by loose or damaged wiring or terminals. Ensure the connections are in good condition and tight. Replace the PCB or the PGRF valve if necessary.

Error Code 61: Combustion Fan Failure

Rheem error code 61 means there is a problem with the combustion fan. To troubleshoot Rheem error code 61, check the motor’s wiring condition and the Molex connections at the PCB. Check if the fan motor moves freely and is not blocked or damaged. Replace the PCB or fan if necessary.

If the fan doesn’t move, then ensure it receives enough voltage. 

If it does, you should check if there is pulse output from it. 

If there is no pulse output, the fan should be replaced. 

However, if there is pulse output from the fan or the fan doesn’t receive enough voltage, the PCB should be replaced.

Error Code 65: Water Volume Control

Rheem error code 65 means there is a problem with the water volume control. While troubleshooting, Rheem error code 65, confirm the unit receives enough water and all wiring connections are in good working condition and tight. Replace the water control assembly or the PCB if necessary.

Data about the water flow can be found in the maintenance information.

Restart the unit and confirm the water control receives enough voltage. If the water control does not receive enough voltage, the reason may be a faulty PCB, and if it does, a defective water control.

Error Code 66: Water By-Pass Control fault

Rheem error code 66 indicates a problem with the water by-pass control. If your Rheem displays an error code 66, confirm the unit receives enough water and all wiring connections are in good working condition and tight. Replace the water control assembly or the PCB if necessary.

Data about the water flow can be found in the maintenance information.

Restart the unit and confirm the water control receives enough voltage. If the water control does not receive enough voltage, the reason may be a faulty PCB, and if it does, a faulty water control.

Error Code 71: Gas Inlet Solenoid Valve

Rheem error code 71 means there is a problem with the gas inlet solenoid valve. To fix the Rheem error code 71, ensure the gas inlet solenoid valve is installed correctly and its wiring harness is tight and not damaged. Replace the PCB or the gas inlet solenoid valve if needed.

Error Code 72: Flame Sensing Device fault

Rheem error code 72 means the flame probes are detecting flames. Check the flame rod’s condition, wiring, and voltage. Ensure it touches the flame. The gas control assembly, the main burner, the flame rod electrodes, or the PCB may need to be replaced.

Follow the same troubleshooting steps and procedures as in error code 51.

Error Code 76: Communication Fault with Remote Control

Rheem error code 76 indicates a failure in communication with the remote control. If your Rheem displays an error code 76, confirm the remote control wiring is in good condition and tight. Replace the remote control, remote control’s wiring, or the PCB if necessary.

Disconnect the remote control, switch the unit on, and open a hot water tap to start it. 

If the unit starts, then the problem is either with the remote control or its wiring. To determine the root cause of the issue, use a jumper wire to connect the remote control to the PCB directly. If the remote works, the issue is with the cable, and if it doesn’t, with the remote control itself.

If the unit doesn’t start after disconnecting the remote, its PCB may need to be replaced.

Error Code 79: Fan Motor Current fault

Rheem error code 79 means there is a problem with the current of the fan motor. Rheem error code 79 can be caused by condensation or moisture accumulation over the fan motor or damaged or loose wiring. Replace the PCB or fan if needed.

Follow the same troubleshooting procedures outlined in error code 61.

Error Code 80 and 81: Gas Cut-off Failure / Extinction Failure

Rheem error codes 80 and 81 indicate a gas cut-off or extinction failure. Ensure the flame rod and gas valve’s wiring are in good working condition, installed correctly, and receive enough voltage. The valves, the main burner, the flame rod electrodes, or the PCB may need to be replaced.

The same troubleshooting steps as in error code 51 should be followed.

Error Code 82: Gas Type Control Data Failure or PCB data failure

Rheem error code 82 means there is a problem with the PCB programming. Ensure the programming chip is suitable for your Rheem model and the altitude the unit is used at. Restart the unit and adjust the high altitude fuel type chip and dip switch. Replace the PCB if the error code 82 persists.

Remember that depending on your altitude, or for every 3,280 feet, a different programming chip is used, and indoor and outdoor units use different chips.

Error code 90: Blocked Flue

Rheem error code 90 indicates a blocked flue. Check the vertical and horizontal vent length and ensure they meet the standards for safe operation. Ensure the heat exchanger flapper is not blocked and moves freely and that the whole vent system is clean and not blocked in some way.

Disconnect the vent structure from the unit. Run the unit and check if the error code persists. 

If the error code goes away, the problem is within the vent system, and if it doesn’t, the PCB or the blower motor should be inspected and replaced if necessary.

Error Code 92 and 93: Neutralizer Abnormality

Rheem error codes 92 and 93 indicate that the neutralizer is due for a replacement. Rheem error codes 92 and 93 are normally found on condensing units. During operation, error code 92 is displayed, and when the unit is switched off, error code 93 is shown. To fix this error, replace the neutralizer.

Error Code 99: Fan Motor is not creating enough ventilation

Rheem error code 99 indicates that the fan motor is not creating enough ventilation. If your Rheem displays error code 99, ensure nothing blocks or obstructs the heat exchanger fins, the exchanger flapper valves, the vent termination, and that the air intake filter is clean.

Rheem error code 99 can simply mean abnormal combustion caused by a blocked or obstructed fan, heat exchanger, air intake, or exhaust venting.

During the troubleshooting, you can remove the vent structure from the unit and try to start it. If the error code disappears, then the problem is within the venting. If not, then the problem could be caused by the blower motor and the PCB.

Error Code P1: Low water flow rate

Rheem error code P1 means the water flow rates have fallen below 0.66 GPM for more than 5 seconds. If your Rheem displays an error code P1 the water flow rates are not sufficient for the main burner ignition. Ensure there is enough water flow, and the cold water inlet and aerator screens are clean.

Error Code IL: Lime build-up inside the heat exchanger

Rheem error code IL indicates a possible scale build-up in the heat exchanger. To fix the Rheem error code IL, drain and flush the heat exchanger as recommended in your owner’s manual, which has been provided by Rheem.

No Error Code

It is possible for a Rheem unit not to heat water, though it has a flowing water supply and does not display any error codes. In this case, there are a few things that you need to check.

  • Ensure the unit is powered.
  • Check the condition of the circuit breakers and power supply inlet fuses.
  • Ensure the unit is installed correctly and the hot and cold water lines are not mixed up, and there is enough water flow.
  • Ensure no parts of the unit are blocked, clogged, or obstructed by debris, foreign objects, or dirt and scale build-ups.
  • Check for bleeding and trapped air.
  • Sometimes this issue can be fixed by simply restarting the unit. Stop the water flow, switch the remote control off, and unplug it. After 10 seconds, plug it back in. After another 20 seconds, switch the remote control on and the water supply too.
  • If the dip 1 switches are switched off, this can also prevent the unit from working properly.
  • A faulty water flow sensor or PCB can also be the reason for your unit not working.

Other Troubleshooting Tankless Water Heater Error Code Guides

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Hubert Miles

I've been conducting home inspections for 17 years. I'm a licensed Home Inspector, Certified Master Inspector (CMI), and FHA 203k Consultant. I started HomeInspectionInsider.com to help people better understand the home inspection process and answer questions about homeownership and home maintenance.

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