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Rheem Tankless Water Heater Error Code 12 (This Should FIX it)

Rheem Tankless Water Heater Error Code 12 (This Should FIX it)

The Rheem tankless water heater allows you to access instant hot water. It is a significant upgrade to the version that uses tanks and delays water supply till it’s hot. But like any other piece of technology, they develop faults. The system detects some of these faults using an error code. The focus today is on error code 12 and what it means.

Failure to detect flame in the burner system is the reason for error code 12 on your Rheem tankless water heater. Common causes include faulty flame sensors and inadequate control valves. That said, there are solutions to these causes that are easily applicable.

Here is a detailed explanation to help you understand why these error codes exist. Moreover, knowing what causes the error allows you to fix it properly. What’s more? You also learn how to prevent errors in the future.

Read: Amana Washer Stuck on Sensing Fill 

Reasons and Fixes for Rheem Tankless Water Heater Error Code 12

Error Code 12 indicates that the system cannot detect flames while heating water.

This error code shows that there is a problem with the flame unit, and this can result from several issues. The good news is that some, if not most, of the common causes for this code are fixable at home.

With the proper guide to the diagnostics steps and the possible solution or fix to this problem, you have all you need to solve this problem.

Here is a comprehensive guide to the common causes of error code 12 and the solutions:

Interrupted Gas Supply

Gas supply to the tankless water heater is the first option when diagnosing the reason for this error code. When there is no supply from the gas tank to the burner, there is no ignition.

Without ignition, there is no flame. When there is no flame or the system doesn’t detect flames, the error code appears. 

It is also the simplest and easiest option to check. You don’t need much expertise to check for this error. Hence, it should be the first place to start.


Diagnosis and Possible Solution

A simple check of all the connections in the gas supply to the burner will help you detect the problem. You can also apply the fix or solution accordingly. Here are the diagnostic steps for this particular issue:

  • Make sure to power off your heater, and disconnect it from the circuit breaker. It is essential to avoid fire accidents.
  • The first step is to check the gas level in the tank and be sure it is sufficient.
  • Next, check the control valve at the top of the tank. Check that the valve is not stuck or clogged on the side.  It supplies the hose that connects to the burner.
  • Check the hose and ensure a gas passage from one end to the other. Straighten the hose if it bends anywhere.
  • If the bending has led to damage, such as cuts on the hose, it is crucial to close the control valve and replace the hose as soon as possible.
  • If the hose is okay, check for clogs in the hose. Clogs prevent gas supply. Wash the inside of the hose with soap and water.

These diagnostic steps should solve the problem if the issue is with the gas supply. However, if the error persists, you can check other possible problems.

Faulty Control Valve

The control valve on the heater ensures gas supply to the heater in adequate pressure and measure. If it is faulty, then there is no supply to the ignition, even if there is a supply from the gas tank. Reassuringly, it is difficult to repair any part of this control valve.

Diagnosis and Possible Solution

Replacement is your only option once the gas control valve becomes faulty. If you are new to this type of issue or situation, you might want to call in an expert to help with the replacement.

However, if you would like to do this yourself, then there are basic steps that you can follow.

Here is a video that guides you on how to remove and replace the gas control valve of your Rheem tankless water heater;

Dirty Flame Sensor

The flame sensor is essential for the data system of your water heater to detect or clear the error code 12. A functional flame sensor presents no problem. However, when dirt or carbon settles on it, flame detection becomes difficult, triggering the error code 12.

However, with some tools, steady hands, and caution, you can run some diagnostic steps and fix this issue.

Diagnosis and Possible Solution

The flame sensor is part of the inner compartment of your heater. Hence, it would be best if you had a bit of expertise and a tool kit for these steps. A flathead screwdriver is a primary tool here. Here are the steps involved:

  • Disconnect the tankless water heater from the power source or the circuit breaker.
  • Using the screwdriver, open the access panel and remove the cover to reveal the inner units.
  • The Flame sensor is usually at the front, just behind the panel cover. Use the manual to confirm the sensor’s location.
  • The flame sensor is usually yellow and has a white base. Pull hard to detach from the floor.
  • After pulling, remove the nuts that hold the sensor in place. The flathead screwdriver is helpful for this step.
  • Gently clean the dust or carbon deposit on the sensor. The sensor’s tip is delicate, hence cleaning gently to avoid damage.
  • If the sensor is dirty and faulty, then the best option is to replace it.

Cleaning the flame sensor solves the problem. However, if you remove the sensor and it is not dirty, it may be faulty.

If your heater is not more than eight months old, it couldn’t have accumulated enough smoke to cause a sensor block. Then replacing the flame sensor might be the next best thing.

However, I’ll advise you to get an expert to check the Rheem tankless water heater for issues before replacing a new flame sensor.

Clogged Venting Pipe

The venting pipe of your tankless water heater is another critical component that helps with the system’s normal functioning. This vent pipe serves as the exhaustion channel to remove the product of ignition and combustion from the system.

One such product is poisonous carbon monoxide. This gas prevents the ignition of flame when it is much. It replaces oxygen that the burner needs for ignition. When this happens, error code 12 appears on your heater.

Diagnosis and Possible Fix

The aim here is to remove the ventilation pipe and remove any clog as a result of smoke accumulation or soot. Hence it is necessary to detach the line from the setup and check independently. Here are the steps involved:

  • Power down the tankless water heater by disconnecting from the power source or circuit breaker.
  • Stop gas supply by locking the gas control valve or the gas tank.
  • Locate the ventilation pipe in the setup. It is usually a 4ft pipe that connects to the chimney system from the burner section.
  • Unscrew the pipe at both ends to disconnect for proper inspection.
  • If there is a clog, remove it using appropriate tools. It would help if you also cleaned the inside of this pipe to keep it neat.

If the issue of a clogged ventilation pipe becomes a problem, then it is necessary to call an expert to help inspect the entire components of the tankless water heater. Frequency shows that there is something wrong with the combustion process. So an expert can help detect the real problem.

Note: there are other possible causes of this error code. But they are not as frequent as those on the list.

One such cause is inadequate gas pressure. This issue arises when there is gas flow, but it does not have enough force to cause ignition. You can use the owner’s manual as a guide to correct the pressure. You should not have this problem if you had an expert to help with the installation.

How to Reset the Rheem Tankless Water Heater?

Your Rheem tankless water heater has a reset button to help clear error codes within the system. It is a necessary step after fixing any diagnostic problem. Sometimes, after applying the appropriate solution, the system does not clear the error. Resetting the heater is the ideal step at this point.

Here are the steps to help locate and use the reset button:

  • Disconnect the gas valve from the heater to ensure safety.
  • Power on the tankless water heater from the circuit breaker.
  • Unscrew the panel cover to access the reset button. You can use the Rheem Tankless Water Heater User manual as a guide to the reset button.
  • Press and hold the reset button for a few seconds. You should hear a confirmation click. Then release the button.
  • Return the cover and firmly reconnect the screws.

These steps should reset the system and clear the error codes

Warranty and Support

Rheem is a global leader in water, heating and cooling products. If your Rheem tankless water heater is till giving you trouble, it’s time to contact Rheem support.

Visit Find a Pro on or Call Rheem Customer Service at 800-621-5622

Email: [email protected]

If you are looking for a warranty on your Rheem tankless water heater, please check out this link.

Rheem generally offers 1-year limited warranty on its products.


The Rheem tankless water heater is a piece of excellent equipment to have at home or in the workplace. Instant provision of hot water makes it ideal for many users.

Clearing the error codes ensures you continue to enjoy this benefit and many others.

The possible fix to clearing these codes are as follows:

  1. Ensure adequate gas supply
  2. Replace the faulty control valve
  3. Clean and maintain the flame sensor
  4. Unclog the ventilation pipe

Diagnose and find the problem; only then can you apply the appropriate fix. Using the fix helps to keep the system functional. So keep the system error-code free, and continue to enjoy the wonders of the modern world.