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Rheem Tankless Water Heater Error Codes

Note: This post may contain affiliate links. This means that at no cost to you, we may receive a small commission for made purchases.

As soon as you spot an error code on your Rheem tankless water heater you may begin to panic as to what it means.

You may not need to contact a qualified specialist to take a look as the error codes can be troubleshot by yourself.

Before you do ask for extra help, find out what the error code means and you may be able to solve it without making a service call. 

In this guide, we will look at where you can find the most current information for your Rheem tankless water heater and look at the error codes (see also ‘AO Smith Tankless Error Codes: Problems And Troubleshooting‘) in detail.

Where You Can Find Current Information For Your Rheem Tankless Water Heater

Of course, the internet is likely your go-to resource for any current information relating to your Rheem tankless water heater.

However, you can also find a wealth of information from the owner’s manual which will be specific to your model of a tankless water heater

Features and error codes may have changed over the years so finding an updated resource is vital to ensure that you know what to do when a fault looks to have occurred. 

The Error Codes On A Rheem Tankless Water Heater

There are many error codes on a Rheem tankless water heater (see also ‘Rheem Water Heater Leaking—What To Do?‘) and their reasons differ vastly.

Knowing what each error code means can save you from having to make a service call and can give you peace of mind about how serious, or not so serious, the problem is. 

If the fault is not so serious then you may be able to fix it yourself and get your water heated soon enough. 

Error Code 00

Reason: Continuous combustion lasting an entire hour

If your Rheem tankless water heater has been running continuously for over an hour you should look to reset the system.

This is relatively simple and can just mean shutting off all your hot water faucets. 

Should you have a circulation pump installed, you can also turn that off or even remove it.

This is largely because the Rheem tankless water heater is not specially designed for continuous running along with a circulation system.

Error Code P1

Reason: Water flow is too low

Some simple issues can result in the water flow being low and you should be able to solve them yourself.

From cleaning the aerators as well as checking the pressure and flow of the water from your hot water faucet to checking then cleaning the water inlet filter on your tankless unit. 

You could increase the flow rate or simply adjust the temperature though if all of these potential solutions fail then there may be a problem with the water flow sensor itself which needs to be replaced.

Error Code 1L

Reason: Build-up of limescale

Performing regular maintenance on your Rheem tankless water heater should prevent error code 1L from occurring.

You may need to flush the system using distilled white vinegar though do be careful as the water would naturally be hot. 

If you do not feel comfortable performing this yourself then you can contact a professional to do it for you.

This particular error code could also be triggered due to the wrong altitude setting if you reside in a high-altitude environment. 

Error Code 05

Reason: Combustion issue

Error code 05 could be triggered by an issue with the air intake or a blockage leading to the vent exhaust.

Whichever issue it is, your unit will likely still be operating and you can solve the error yourself.

For the air intake, remove then clean the air inlet filter and clean both the heat exchanger fins and the combustion air fan.

You should also check that the fan motor looks to be working properly. 

The vent exhaust can be a little more complicated to solve though you should start by simply checking and removing any obstructions to the exhaust vent.

Ensure that the vent pipes are connected to the vent termination and that the venting meets various requirements (such as lengths, diameter, the construction material).

The venting should also be adequately sealed and you may want it inspected by an expert. 

Error Code 10

Error Code 10

Reason: A false flame has been detected

A false flame being detected may also necessitate a service call, yet there are certain steps to take first.

Check that the vents are connected properly with no obstructions, the correct materials should have been used for the installation. 

There should definitely be no obstructions for the exhaust termination and flue inlet, check that both have the right amount of clearance space too.

Finally, the heat exchanger should be checked to make sure that the fins, fan, and air intake all appear to be clean. 

Error Code 11

Reason: Failure to ignite

Knowing that the ignition has failed is straightforward enough yet finding out why can prove difficult.

Check for an operational igniter and for any damage to the igniter wiring harness.

The gas solenoid valves should also be in decent condition though they may be open or have short-circuited. 

There should also be gas to the unit with the valves fully turned on and the unit itself should have the correct gas volume and pressure which can be checked by a specialist. 

Error Code 12

Reason: Flame failure

A flame failure is a little different from a failure to ignite and will likely require a specialist to take a look.

Before they arrive, you can check that the gas valves are all open with gas actually reaching the unit properly.

The flame rod wire should be connected and it should be free of any carbon build-up. 

Error Code 13

Reason: Poor/Improper Combustion due to the flame rod

Problems with the combustion can also be caused by the flame rod though this should only display on an indoor Rheem tankless water heater system.

Double-check that the exhaust venting and air intake are both installed properly and look to be in good working order.

For instance, the exhaust cannot be recirculated back into the unit’s fresh air intake. 

Take the time to remove any obstructions from the venting as well as where the vent terminates outside.

You may also want to ensure that the altitude settings are correct for your location. The blower motor and flame rod FL-25 should also be checked.

Error Code 14

Reason: Overheat Wrap Fault

Obstructions could also result in error code 14 though if you cannot find one then consult a professional to troubleshoot the fault.

There are certain steps you can take which include checking the high and low fire manifold pressures and that the right type of gas is used. 

The heat exchanger may also be suffering from separations or cracks.

If you do think that there are obstructions and foreign objects then they may be in the combustion chamber, the exhaust piping, the vent terminal, or preventing sufficient airflow from coming into the unit itself.

Error Code 15

Reason: Boiling Safety Device

Should the heat exchanger keep a temperature of 207 °F for a period of longer than 15 seconds then error code 15 may occur.

You may need to ensure that the inlet valve is completely open and there are no obstructions to the cold water inlet pipe.

You could try to inspect the heat exchanger thermistor or flush the unit and remove any build-up of limescale (see also ‘ What Is Limescale?’ ).

Error Code 16

Reason: Too High Outlet Water Temperature

A similar problem can be the outlet water temperature being beyond the remote thermostat setting.

This fault may be caused by blockages to the heat exchanger, limescale build-up, or even a combination of the two.

Again, there should be no obstructions to the airflow and the vent terminal should be clear.

You could also check the outlet thermistor, heat exchanger thermistor, and the gas valve are all in good working order. 

Error Code 21

Reason: Air Intake Filter Switch Malfunction

This error can be checked via two methods; checking that the front panel is installed properly and that the air filter door looks to be sealed correctly. 

Error Code 24

Error Code 24

Reason: Switch Malfunction

Troubleshooting for a switch malfunction could mean turning it off and on again by forcing a reset on the control board, all you need to do for that is press the MIN and MAX button.

You can also try shutting off the water and then disconnect the remote control. 

If that does not work then you could confirm that the tankless unit is correctly grounded.

If none of these measures works then call in a professional to replace the control board (also known as a PCB).

Error Code 29

Reason: Insufficient Heat Exchanger Temperature

If the temperature is insufficiently low on the heat exchanger then you should receive error code 29 on your mid-efficiency or condensing unit.

That may be the cause but it could also be due to a clogged neutralizer on the condensing unit and you can clean the air inlet screen and the heat exchanger fins. 

For a condensing unit, confirm that the drain line is not blocked, and the shipping cap is removed.

The neutralizer drainage ports should be clear and the neutralizer drainage line (see also ‘7 Awesome Tips For Sewer And Drainage Line Cleaning‘) outside the unit must be clear too. 

Error Code 31

Reason: Inlet Thermistor

A fault with the water inlet temperature sensor would result in error code 31 and you may want to check certain elements of your Rheem tankless water heater yourself.

These can include checking for damage to the thermistor wiring and to check the thermistor for the right number of ohms.

The thermistor should also be free of any limescale build-up and may require a clean.  

Error Code 33

Reason: Outlet Thermistor

There could also be a problem with the outlet thermistor which is the outgoing water temperature sensor.

Before you make a service call, this is a good time to check the thermistor wiring for any damage or the thermistor for a limescale build-up.

You can also check that the ohms are correct though your sensor may need to be replaced. 

Error Code 34

Reason: Ambient Thermistor

Another fault could occur with your ambient thermistor which is the combustion air temperature sensor.

Again, inspect the thermistor wiring for any damage and this is an ideal time to check then clean the ambient thermistor itself and perform a check for the correct number of ohms. 

If there are air flow restrictions surrounding the unit or obstructions at the vent terminal then this could also result in an error code 34.

If checking that the fan blade is tightly fitted on the motor shaft and spinning freely does not work then you may want to replace the sensor. 

Error Code 35

Reason: Improper Thermistor Connection

Error code 35 should require you to inspect each of the thermistor connections to the control board as they should all be securely connected. 

Error Code 51

Reason: Gas Control Valve

Before you make a service call for error code 51, check for any loose/damaged terminals on the wiring harness for the gas control valve.

Error Code 52

Reason: Faulty Modulating Solenoid Valve

Similarly to error code 51, you may want to check that the terminals on the gas solenoid valve wiring harness are not loose or damaged.

If they are then you may want to contact a specialist to take a look.

Error Code 61

Reason: Combustion Fan Failure

Error code 61 should mean that you should check the fan motor, specifically that it turns freely as even a small restriction could be the problem.

There may also be loose or damaged connections attributed to the motor wiring harness that could be the true cause of the error code.

Error Code 65

Error Code 65

Reason: Water Control Valve Error

Error code 65 may necessitate calling out a specialist yet you can check for a fault yourself.

This may be that the water control valve has the incorrect voltage or that there are loose/damaged connections on the water control valve’s wiring harness.

Error Code 66

Reason: Water Bypass Valve Error

To troubleshoot this error code, confirm that the water bypass solenoid has the right voltage going through it.

Just like error code 65, you should also inspect the tankless water heater for any loose or damaged connections on the wiring harness of the water bypass valve.

Error Code 71

Reason: Gas Control Valve Error

There could also be loose or damaged connections on the wiring harness for the gas control valve which could set off error code 71.

This is another fault that you may want to check with a professional. 

Error Code 72

Reason: False Flame Detected

A false flame can be attributed to the flame rod (or rods) touching the flame whenever the unit fires.

There may also be foreign material or obstructions in the burner chamber so check that there are no blockages for the flame at the flame rod (or rods). 

The wiring for the flame rod may also be damaged, and the flame rod needs to have sufficient voltage.

You may even want to take out the flame to check it and clean it with some steel wool. 

Error Code 76

Reason: Remote Control Communication Fault

A communication error with the remote control can result in error code 76.

You may want to do a service call though you can check the wiring on the remote control yourself, specifically for any loose or damaged connections. 

Try connecting the remote control to the remote connection directly which is at the bottom of the water heater, though the cable may be the problem and could be replaced.

Finally, unplug the water heater, disconnect the remote control then plug the heater back in and test it without the remote control. 

Error Code 79

Reason: Current fault with the Blower Motor

Error code 79 should indicate that there is a problem with the current heading to the blower motor and you may want to do a service call.

However, before you do that, confirm that the blower motor is turning freely as it may still operate with a small restriction. 

The fan motor should also have the right voltage and may be water damaged due to condensation.

It may be the case that both the fan motor and the control board need to be replaced. 

Error Code 80 & 81

Reason: Gas Control Valve Fault

Both error codes 80 and 81 suggest a problem with the gas cut-off valve.

You could ohm out all the solenoid valves, inspect the flame rods for the correct voltage, or even replace the gas control valve altogether.

Error Code 82

Reason: The Control Board is not programmed

There may be a fundamental issue with your control board that it is not programmed properly or that the wrong gas type has been entered.

Check that the program chip is properly installed and that the control board recognizes the right type of gas, which could be propane or natural gas. 

Error Code 90

Reason: Blocked Flue/Air Intake

An error code 90 typically indicates that there are blockages in the heat exchanger or blower motor. If there are none, check the exhaust and inlet flue for any blockages.

Error Code 92 & 93

Reason: Neutralizer Replacement Required

The neutralizer may need to be replaced and if it does then error codes 92 and 93 may be set off.

However, both codes would only be displayed on a condensing tankless water heater.

Specifically, error code 92 would display when the water heater is continuing to operate and is a warning to replace the neutralizer soon.

Error code 93 should show when the unit has shown down and is a further warning to replace the neutralizer.

Error Code 99

Error Code 99

Reason: Blower Motor Cannot Vent

Error code 99 can indicate that there is a blocked flue fault that is causing the blower motor to be unable to vent.

First, check the exhaust and venting flue for any blockages. There may also be blockages in the heat exchanger which should be removed.

Finally, check for blockages in the inlet filter and combustion fan. 

No Error Code Displayed

Reason: Several reasons

There are different reasons for why the water would flow through the water heater, but nothing actually occurs, specifically, that the water is not heated.

You could check the power and if the unit is actually plugged in which is a good idea after a tripped or turned off circuit breaker (see also our guide on how to match wires size with circuit breaker ).

The inlet fuses to the power supply should also be in working order and ensure that the hot and cold water lines are not crossed following the installation. 

The minimum flow rate should also be met to make sure that the tankless water heater fires.

There are further elements that you can check including that the inlet water supply filter is clean.

Pipe dope may also cause a blockage inside the water flow control turbine.

This may also be the ideal time to perform a bleed-over which requires the cold water supply valve to the water heater to be turned off.

Open the hot water faucets in your home and if there is a steady flow of water from those faucets then you may have a bleed-over problem which would require a service call. 

Final Thoughts

Learning the error codes of your Rheem tankless water heater may seem exhaustive yet can prove to save you money.

If you can solve a problem or a fault without having to make a service call then, not only will you learn more about how the water heater works, but you will likely save yourself some money. 

Performing regular maintenance can also save you from having to solve any problems from occurring in the first place such as cleaning the filters, the air fan, and the heat exchanger fins. 

Frequently Asked Questions

What Is The Most Common Problem Associated With A Water Heater?

The most typical problem with a water heater tends to be a water leak which can be common due to how many components come in contact with water.

Moisture can also build up in the water heater and the unit will also be connected to the plumbing system.

Water can corrode your water heater and result in tiny fractures or cracks that will inevitably lead to a leak

What Are The Signs To Look Out For In A Failing Water Heater?

If your water heater is old then you may want to pay more attention to the warning signs that it may be failing.

These issues can include water leaks, that it runs out of hot water quickly, and suffers from an inconsistent water temperature.

If the water pressure is low, the water heater is making strange noises, and keeps on coming up with a variety of error codes then it may be time to replace it.

author avatar
Anthony Barnes
Anthony Barnes is the founder of Water Heater Hub and a second-generation plumber by profession. Before developing Water Heater Hub, Anthony Barnes was a full-time plumber, and he has undertaken a wide variety of projects over the decades. As a second-generation plumber, it was easy for Anthony to get used to the technicalities of all from a tender age


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