Rheem Tankless Water Heater Code 12 [Reasons + Solutions]

Rheem tankless water heater code 12 is an error of the system indicating flame failure. Malfunctioning of the flame rod is the major reason for flame failure in the heater. Either the flame rod is clogged by carbon buildup or might be broken. 

rheem tankless water heater code 12

Plus, interruption in the gas supply, power outage, poor airflow due to blocked vents, and bad gas control valve are some leading factors when your tankless water heater cannot detect a flame and pop-up error code 12. 

Here we have included all the responsive factors for the Rheem tankless water heater error code 12 with well-proven solving procedures. Scroll down!

What Does Error Code 12 Mean On A Rheem Tankless Water Heater

The error code 12 on the Rheem tankless tells you there is no flame or flame detection problem in the unit. That means the flame sensor fails to detect any flame in the burner box. 

Then as diagnostic information, the control panel or remote controller displays code 12. When the error occurs, the gas supply turns off, stops the water heating operation, and as a result, you won’t get hot water

How To Fix Error Code 12 On Rheem Tankless Water Heater [5 Easy Solutions]

Before moving to the details troubleshooting steps, let’s have a glimpse at the below table highlighting the probable reasons for error code 12 with quick fixes:

Probable ReasonsPossible Solutions

Gas Supply Issue
Check if the gas valve is closed. Ensure the correct gas pressure. Bleed the air from gas lines. Ensure gas lines are sized according to the BTU rating.

Power Supply Issue
Make sure the circuit breaker is turned on. Fix the loose wiring connection to the unit. Ensure the appliance is properly grounded. Replace the tripped circuit breaker.

Clogged, or broken flame rods
Clean the flame rods using fine sandpaper of 100 grit. Replace the broken flame rod.

Poor airflow
Clean the air intake filter. Inspect the exhaust vent for clogs. Ensure the condensation trap is blockage-free and installed properly
Bad gas control valveReplace the bad gas control valve.

Dealing with Rheem tankless water heater code 12 will seem easy as long as you will follow the below step-by-step guide:

1. Interruption In The Gas Supply

If error 12 is popping up on your Rheem tankless water heater control panel, an interruption in the gas supply is the major reason behind this problem. To keep functioning the water heater, a fluent gas supply is a must. 

Otherwise, the heater failed to ignite due to a lack of gas supply, resulting in no flame. Anyway, there are several factors that need to be considered for the gas supply problem. 

Let’s glance at all those responsive factors with effective solution methods:

i). The Gas Valve is Closed

First up, you have to check the gas valve to see if it is opened. You will find a red handle on the valve controlling the gas flow.

Ensure the gas valve is in the “on” position or the handle is in line with the direction of flow of the pipe.

ii). Clogged Or Bent Gas Hose

If the gas is not flowing, it is highly possible that either the gas hose is clogged by debris or gets bent. At times, the air in the gas hose might be responsible for preventing the gas from flowing. 


  • You need to clean the gas hose with soapy water. But before that, make sure you have turned off the gas supply. 
  • Besides, check the hose to see if it is kinked. Straighten the bent hose.
  • Bleed the air from the gas lines. 

iii). Insufficient Gas Pressure

Rheem tankless water heater flame failure might occur due to a lack of proper gas pressure. To continue the water heater operation, the gas pressure for both inlet and manifold should be in the following recommended measurement. 

Gas PressureNatural Gass LP Gas
Inlet Gas Pressure5.0″ W.C to 10.5″ W.C9.5″ W.C to 13.0″ W.C
Manifold Gas Pressure2.52+ fan pressure (WC)7.36+ fan pressure (WC)


Monitor the gas pressure on a water heater. If the high and low gas pressure is not sufficient, adjust the gas pressure regulator according to the above-mentioned measurement. 

It will be better to call a gas technician to handle the task.

iv). Undersized gas lines for the BTU Load

If the gas line, meter, and/or regulator are not sized properly, it is one of the responsive facts in the case of gas supply interruption. An undersized gas line causes incomplete combustion, leading to the flame failure issue. 


You have to check if the gas lines are sized correctly to support the BTU load of your tankless water heater. If not, you need to size it. 

For this, you will require the correct gas line size information according to the BTU rating for a tankless water heater. 

Thoroughly read this PDF file for better understanding, or call a professional to complete the task. 

2. Power Supply Issue

Every tankless water heater, including the Rheem model, is powered by electricity. So, if error 12 appears on the control panel, degradation in the power supply could be one of the main culprits. 

The major reasons for the power outage in your tankless water heater include:

  • Tripped circuit breaker
  • The appliance is not grounded properly
  • Power supply issue due to sudden voltage drop
  • Loose or damaged wiring harnesses on the unit and PC board.


  • First, check the circuit breaker of your tankless water heater. Ensure it is in the “On’ position.
  • Then, look for a loose wiring connection on the water heater and fix it. It will be better to disconnect and reconnect all wiring harnesses on the unit and also the PC board. Replace the damaged ones.
  • Check the power supply for voltage drops. Ensure the appliance is grounded correctly.
  • Once done, turn off your water heater and let it unplugged for 5 minutes. Then, turn the water heater back on.
  • If the issue persists from constant circuit breaker tripping, call an electrician.

3. Clogged Or Broken Flame Rods

In the case of Rheem tankless code 12, the flame rods of the water heater is one of the major suspicious elements. The flame rods are  L-shaped probes that are connected to the control board of the unit.  

Detecting flame after igniting and continuing to inflow the gas and air mixture is the main function of the flame rods.

But due to long usage, the flame rod might get clogged by dirt or carbon buildup. When the flame rods are covered by carbon, it will fail to detect the flame and lead to the 12 error code.

Anyway, in this troubleshooting stage, you will need to inspect the flame rod to clean the carbon deposits. 

How To Clean Flame Rods In Rheem Tankless Water Heater

Here are the needed steps to assist in finding the Rheem tankless water heater flame rod location, including the cleaning procedures:

i). Initial Disassembly

First, turn off the power supply of the heater or shut off the circuit breaker that controls the heater. Also, shut off the gas supply to the unit.

ii). Disassemble The Internal Parts

Then, take off the front panel by unscrewing the screws around the unit. Next, remove the printed circuit board and let it hang. 

In the same way, remove the gas supply line and pull the gas pipes out. Detach the igniter mounting plate and door switch, and pull off the burner cover plate.  

iii). Locate The Flame Rods

Unscrew the screw holding the burner assembly, and pull the assembly out of the bracket. You will find about five flame rods on the wall of the burner assembly. If you are confused about the flame rod, see the picture below

flame rods

You can remove the rods by removing the nuts that hold the flame sensor in the burner house. 

iv). Clean The Flame Rods

Now, take fine sandpaper of 100 grit or steel wool to clean the rods. Rub the rods to clean carbon deposits.

clean the flame rods

If the deposits are grimier, use rubbing alcohol or acetone for better cleaning. Wipe down the loose specks of dirt using a paper towel. In case the flame rods are damaged, it’s better to replace them. 

Once the flame rod is re-installed, turn on the power supply and allow water into the system. 

4. Blocked Water Heater Venting

Recirculation of the combustion gas inside of the heater is one of the major reasons for flame failure in the Rheem tankless water heater.

Due to poor air supply, the combustion gas is stuck and fails to remove. This abnormal combustion interrupts the flame detection operation and triggers the error code 12. 

Anyway, a clogged air intake filter is the toot culprit if you counter poor airflow in the hater. Also, you will find a 3 or 4 feet vent pipe on the top of the unit connected with your chimney flue system. 

If the vent pipe is blocked by dirt or debris, it won’t allow air to get inside the heater. 

Plus, improper vent length and incorrect installation of the condensate collar are highly responsible for the problem of combustion specifications. 


To ensure proper airflow in your water heater, follow the below step-by-step guide:

i). Clean The Air Intake Filter

First, press the power button on the control and turn off the heater. Disconnect the power supply and turn off the gas supply to the heater. 

Then, remove the front cover to expose the internal parts. Locate the air intake from the top of the tankless water heater frame. See the picture below to locate the air intake filter.

clean the air intake filter

Remove the screw holding the air intake filter in place and pull the filter out of the air intake adapter. 

Now, clean the filter with a toothbrush and then rinse it under the running water. Once cleaning is done, reinstall the filter the same way.

ii). Unclog The Condensate Trap & Exhaust Vent Pipe

Inspect the vent pipe from the top of the water heater for a clog. You will find stuck condensate debris, dust, oil, etc., at the vent pipe. Clean it in the following way:

  • Take a vacuum cleaner or a damp towel and remove the build-up blockages from intake pipe openings. Besides, ensure the vent length meets recommendations for the venting size.
  • Also, inspect the condensate trap for any degradation. The corrosion or excessive condensate build-up on the trap might be causing a condensate backup in the exhaust line, leading to poor airflow. 
  • At this time, ensure the condensation collar is installed properly. 

If all the tasks seem tough for you, it’s better to contact a qualified service technician to inspect the water heater venting system.

5. Bad Gas Control Valve

If all the above-mentioned troubleshooting steps fail to prove well, the bad gas control valve could be the last probable reason for popping up Rheem tankless error 12. 

The gas control valve is the essential part of the water heater, which controls or regulates the gas flow and optimally performs combustion. 

If this part goes bad, the gas supply stops, and you will not get any flame. Either the gas valve wires are not secured or get damaged. The only solution is to replace the gas control valve to fix this issue. 

How To Replace The Gas Control Valve In Rheem Tankless Water Heater

Follow the below DIY methods to replace a gas control valve on a Rheem water heater:

  • First, disconnect the power supply of the water heater and close the gas shut-off valve. Then turn off all the water valves. 
  • After that, loosen the screws around the unit and remove the front panel to explore the internal parts. Locate the gas valve and disconnect the wiring connection to the valve. The below-marked picture will assist you in this case.
  • Next, loosen the nuts.
  • Then, you will find two screws connecting the gas valve on the bracket. Remove the screws and disconnect the gas valve from the cabinet.
  • Now take the new valve and install it at the same place. After installing, ensure a secured wiring connection to the valve and also check for gas leaks. Look at the o-rings for damage and replace damaged o-rings.
  • Finally, reassemble the water heater. Turn on the power supply, and the gas shut-off valve and open the water valve.

After all the above attempts, the flame failure issue from your Rheem tankless water heater should be erased. 

How To Clear The Error Code 12 On Rheem Tankless Water Heater

Once you have performed all the above troubleshooting steps and fixed the culprits, next, you have to reset the unit to clear the error code. To reset your Rheem tankless water heater, follow the below steps: 

  • First, you have to remove the front panel cover. Unscrew the screws around the unit and pull out the front cover.
  • Then, find the reset button from the inside. You may find it behind the insulation. 
  • Press and hold the reset button inwards until you will hear the click sound.
  • Once resetting is done, the error code will be cleared. Connect the panel covers back into its place.  


Do my Rheem tankless water heaters need to be serviced every year?

You should concentrate on the proper maintenance of your tankless water heater. So it’s better to service your water heater at least once a year. It will help to detect the mini problem and can be solved before any large damage.

What maintenance does the Rheem tankless water heater require?

If you are operating your Rheem tankless water heater regularly, you shouldn’t miss some of the maintenance tasks for the unit. That includes descaling and flushing out the unit, and cleaning the water line and the air filter to ensure fluent airflow. 

Bottom Lines!

Hopefully, now you know the reasons behind Rheem’s tankless water heater code 12 and how to fix it. So, whenever, you notice the error code 12 on your Rheem tankless water display, simply follow our guide and fix it in no time.

You can let us know if you have any more struggles with your Rheem tankless water in the comment box. We will be back with the best solutions.

Note: You can also read the guide on why my Rheem water heater beeping and how to fix it.

About William

William is the founder of Fireplacehubs.com. He has real life practical skills in fixing smoker & heating appliance issues. He loves to share his knowledge & helps others in fixing their appliances & saving their money. William firmly believes that anyone can repair his or her unit with the correct guidance & knowledge. See more at about us.

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