how To Fix Rheem Tankless Water heater Code 12

Rheem Tankless Water Heater Code 12 (Flame Failure)- [Solved!]

Rheem Tankless Water Heater Code 12 means flame failure. The flame sensor fails to detect the flame due to: 

  • A dirty or defective flame sensor
  • Improper gas supply
  • Insufficient gas pressure
  • Gas lines are too small for the BTU load of the unit
  • Clogged venting

Throughout this guideline, I will walk you through each troubleshooting step to reset the fault code 12. 

Let’s dive in. 

Relevant: Rheem Tankless Water Heater Error Codes 

Table of Contents

Rheem Tankless Water Heater Code 12: 6 Troubleshooting Steps

This chapter will disclose all the reasons that cause your water heater to pop out the error code 12. 

01. Dirty Flame Sensor

Flame Sensor Rod
Flame Sensor Rod

If you own the Rheem Tankless Water Heater for a long time, the flame sensor of the heater will get dirty by dirt or carbon buildup. When its probe gets covered with carbon deposits, it fails to detect the flame. Consequently, your control panel pops up the error code 12.  

Note: A dirty flame sensor will not be an issue if the water heater is just 6 months old. In this case, other reasons might cause this problem. Stay with us to know those reasons. However, it doesn’t take a fortune or a lot of time to inspect the flame sensor. 

How To Fix: 

Cleaning the Rheem Tankless Water Heater flame sensor will be an easy fix to this issue. Fortunately, I have published an article on Rheem Tankless Water Heater Flame Rod Cleaning. That article will walk you through 7 simple steps to clean the flame sensor like a pro.

02. Gas Supply Issue

If the gas doesn’t flow from the tank to the burner, you can’t ignite the water heater. When you can’t ignite, how is it possible to get a flame? The gas will not flow through the hose if the gas valve on the tank is closed. Besides, a clogged or kinked hose can also prevent the gas from flowing. Lastly, insufficient fuel in the tank is also responsible for not getting a flame on your water heater. 

How To Fix:

Make sure the valve on the gas tank is open. Next, go and check the gas hose for a clog. If it gets blocked, release the hindrance by washing away the gas hose with soapy water. Before doing that, make sure to turn the gas off. On the other hand, straighten the hose if it gets bent. Finally, ensure there is sufficient gas in the gas tank. 

03. Insufficient Gas Pressure

Without proper gas pressure, you can’t operate the water heater or get flames from it. Here gas pressure indicates both the inlet gas pressure and manifold gas pressure. For proper water heater operation, the minimum inlet gas pressure should be 5.3″ WC for natural gas pressure or 11″ WC for LP gas. 

When it comes to manifold gas pressure, it should be 2.52+ fan pressure (WC) for natural gas. If you power your Rheem water heater by LP, the manifold pressure should be 7.36+ fan pressure (WC). If the gas pressure and the manifold gas pressure are improper, you must adjust them based on the fuel type. 

How To Fix: 

You can’t tell or determine whether there is sufficient gas pressure or not without testing it. If you don’t know how to check the gas pressure on a water heater, check out this article. Alternatively, you can watch the below tutorial from Rheem to check the high and low gas pressure. 

The proper range of inlet gas pressure is between 4.0″ to 10.5″ w.c. for natural gas. On the other hand, the correct inlet gas pressure for LP should be between 8.0″ to 14.0″ w.c. So, adjust the gas pressure according to this measurement based on the fuel you use to power your water heater. 

04. Gas Lines Too Small for the BTU Load

It’s indispensable that the gas supply lines should be properly sized to support the BTU load of your water heater. A small gas supply line can cause incomplete combustion and lets your water heater trigger the error code 12. An undersized gas line isn’t only responsible for flame failure but also causes carbon mono-oxide.  

Therefore, it may cause the burners to sooth, pilot lights and burners will go out intermittently, and form condensate in the heat exchanger. To avoid these issues, you must size the gas supply line according to the BTU rating of your water heater. 

How To Fix: 

What will be the correct gas line size will depend on the BTU rating of your water heater, the other appliances, and where you will install them on each branch from the meter and regulator. 

Two methods, the longest length method or the branch length method, will help you determine what size pipe you need for your tankless water heater. We recommend you follow the below charts to size the gas line correctly. 

Gas Pipe Sizing For Water Heater


Gas Pipe System Sizing

If you find these charts difficult to understand, check out this PDF file on Information for properly sizing gas lines for use with Tankless Water Heater. 

We recommend you call a professional to do this task for you. 

05. The Venting Gets Clogged

A clogged or blocked vent pipe of your water heater may also trigger the error code 12. This pipe may get clogged by dirt, debris, and insects if you don’t clean it for a long time. If the vent pipe gets blocked, it will release carbon monoxide gas into your house. 

How To Fix: 

First, identify the vent pipe of your Rheem water heater. Typically, it’s the 3 or 4 feet pipe that extends from the top of your natural gas water heater connected with your chimney flue system. 

Once you detect the vent pipe, it’s time to turn off the water heater and gas supply. Then, unscrew a section of the vent pipe and visually inspect it for a clog. You can run a plumbing snake through the pipe to release the blockage.

How To Reset Rheem Tankless Water Heater?

The following step-by-step guide will help you reset the Rheem Tankless Water Heater with ease: 

  • Shut off both electric and gas supply to the water heater.
  • Detach the power cord from the electric outlet
  • Leave the water heater for several minutes
  • Again, plug the unit into the electric outlet and turn the gas supply on. It should reset your tankless water heater.


Your Rheem Tankless Water Heater will trigger the error code 12 if the flame sensor gets dirty or fails to sense the flame. So, cleaning the flame sensor or rod will be an easy fix for the issue.  

However, your tankless water heater may also display this error message due to gas supply issue, insufficient gas pressure, and a defective gas control valve. Fortunately, we mentioned every troubleshooting step to solve those problems. 

On the other hand, if you fail to clear the error code 12 after trying all the steps, don’t hesitate to reach a professional.


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