How To Fix Rheem Tankless Water Heater Code 12: 5 Solutions 

Imagine this, you step into your shower and dream about a relaxing warm shower. Instead, you are met with a cold sprinkle and an error code 12 flashing on your Rheem tankless water heater.

Before diving into the fixes for this error code, you should know that this problem is common in various Rheem tankless heater models, including:

  • RTGH-65D
  • RTGH-84D
  • RTGH-94D
  • RTG-64
  • RTG-85
  • RTE-13
  • RTE-18
  • RTE-27
  • Performance Plus 5
  • Performance Plus 6
  • Professional GPH65
  • Professional GPH84
  • Classic 65
  • Classic 84
  • Condensing 5
  • Condensing 6

What Is Error Code 12 on a Rheem Tankless Water Heater?

Encountering the error code 12 on your Rheem water heater leaves you confused and desperate for warm water. Here’s a breakdown of this frustrating problem:

  • Meaning: A flashing error code 12 indicates a flame failure. Hot water on demand requires a continuous flame inside your tankless water heater. This issue is often associated with ignition malfunction or heat exchange system problems.
  • Safety Feature: While inconvenient and frustrating, this error code 12 protects you from serious complications. A malfunctioning flame can lead to gas leaks, overheating, or even potential explosions. When your heater activates this safety alarm, it shuts down operation.
  • Impact on Use: This error code prevents your water heater from providing hot water. So, your shower, laundry, and dishwashing plans might be on hold.
  • Warning Signs: Before getting the flashing error code, you may notice a weak flame and hot water flow. Then, both of them become absent.
  • Importance of Addressing: When this code appears on your Rheem tankless water heater, you should promptly take action. Otherwise, you won’t have a hot water supply and you might encounter serious safety hazards like gas leaks.

Why Does Your Rheem Tankless Water Heater Show Error Code 12?

Flame failure in a Rheem tankless water heater may occur due to various reasons. Understanding these potential causes can empower you to troubleshoot or seek the right repairs. Here are some common culprits behind error code 12:

1. Fuel Delivery Problems

When your Rheem tankless water heater blinks the error code 12, you would typically blame low inlet gas pressure. If your heater has no sufficient natural gas flowing through its gas lines, it faces difficulty igniting or maintaining a proper flame.

Fuel delivery problems happen when there is a partially closed gas control valve, Also instabilities in the main gas supply can lead to a low gas pressure. On top of that, any blockages within the heater’s gas line contribute to the problem.

2. Problematic Flame Sensor

The flame rod or sensor of your Rheem tankless water heater ensures the presence of a steady and proper flame. Its role is to detect the presence of a flame. You can find this metal rod within the burner assembly.

By acting like a flame sensor, the rod does its job. Once the flame heats the unit’s air, it creates ions. The rod uses a tiny electrical current to detect these ions. This process can confirm the presence of a proper flame.

However, if it fails to detect heat, it automatically shuts down the gas supply. This sensor may malfunction when it gets dirty with carbon buildup, dirt, and debris. In some cases, this malfunctioning sensor may send a false 12 error code, even when a flame is present.

3. Malfunctioning Filter

Your Rheem tankless water heater has one filter on the inlet water line. This inlet filter prevents sediment and debris in your water from entering the unit and potentially clogging the heat exchanger. A blocked filter disrupts the proper flow of water and air. This can lead to an unstable flame and trigger error code 12.

4. Cracked Combustion Chamber

The combustion chamber of your Rheem tankless water tank acts as a small power plant. In this chamber, the combustion gas is burned. This controlled burning generates heat. that warms your water supply.

The combustion chamber comes with a tight seal to secure its function. Cracks in this chamber lead to unbalanced gas flow and unclean burning. A cracked chamber may hinder the functionality of the regulator, which controls the gas pressure.

Leaving a cracked combustion chamber unattended triggers the Rheem tankless water heater code 12. It also leads to shutting the unit down for safety.

5. Blocked Exhaust Vents

Exhaust vents of your Rheem tankless water heater remove harmful combustion gasses, like carbon monoxide from your home. These vents get blocked for various reasons, such as debris, leaves, and even bird nests.

When you have a clogged vent, these glasses remain trapped inside the unit. Their presence disrupts the clean burning process inside the heater. This may lead to triggering error code 12 as a safety measure.

How Do You Reset a Rheem Tankless Water Heater?

Before investigating internal components, like the flame rod or the combustion chamber, you should try resetting your water tank. This solution may help in the event of a temporary glitch causing error code 12, like a sudden power supply problem.

  1. Turn off the gas supply at the main gas shut-off valve.
  2. Turn off the unit’s power supply at the circuit breaker box to prevent electricity from reaching components like the control board and the sensors. If your Rheem water heater comes with a power plug, you should unplug it.
  3. Wait for 2 minutes and power the heater back on.

If resetting doesn’t clear the error code 12, it’s time for advanced repairs with potential causes.

How Do You Fix Rheem Error Code 12?

Before attempting to DIY troubleshoot the internal components of your Rheem tankless water heart, you should always turn off the appliance at the gas shut-off valve. Also, check to confirm there are no gas leaks around the gas connections or fixtures.

If you sense any leaks, it’s safest to consult a qualified repair technician for fixes involving gas lines.

1. How To Fix Fuel Delivery Problems

In some cases, fuel delivery issues can be tackled without calling in a pro. Here’s what to do before making the call:

  1. Ensure that the gas shut-off valve is fully open.
  2. Check with your gas provider that there are no temporary gas supply fluctuations in your area. These uncommon and sudden dips can cause low pressure on the heater.
  3. While blockages due to debris are less common, they can restrict gas flow. If you suspect a clog, it’s best to call a qualified technician to diagnose and fix the issue safely.

2. How To Fix Problematic Flame Sensor

Troubleshooting an unclean flame sensor can be a simple task. Here’s what to do:

  1. Disconnect the gas supply and electrical power from the water heater. Wait until the unit cools down.
  2. Unscrew the access panel to reveal the heater’s internals.
  3. Look for a thin stainless steel rod within the burner assembly.
  4. Use a nut driver to detach and remove the rod from its wiring harness.
  5. You can use fine-grit sandpaper, soft steel wool, a scotch brite sponge, or a commercial electrical contact cleaner to remove carbon deposits or other buildups.
  6. If the sensor appears cracked or damaged, you should dispose of it and install a new one.

3. How To Fix Malfunctioning Filter

Ignoring a clogged filter can restrict water flow and lead to error code 12. Here’s how to address it:

  1. After disconnecting the power from your Rheem tankless water heater, look for the inlet filter.
  2. Find it on the cold water supply line entering the unit, precisely where the pipe connects to the heater.
  3. Remove the filter from its housing.
  4. You should either clean or replace the reusable filter. To remove clogs, rinse it under running tap water. If the filter appears damaged or severely clogged, get another one.

4.  How To Fix Cracked Combustion Chamber

Troubleshooting a cracked combustion chamber isn’t a DIY task. The chamber has a complex nature and safety risks are associated with it. Therefore, you’ll need a professional to inspect the chamber, apply repairs or replace it.

5. How To Fix Blocked Exhaust Vents

In most cases, clearing obstructions from a blocked vent is a simple task. Here’s what to do if you suspect a blocked vent:

  1. Check the vent’s outside opening. You can find it on your roof or exterior wall.
  2. Examine the vent outlet from the ground and check for visible clogs.
  3. Use a broom to remove reachable blockages.
  4. Extensive or out-of-reach blockages require a qualified technician to thoroughly inspect and clean the entire exhaust venting system.


Now that you have a clear understanding of your Rheem tankless water heater code 12, the troubleshooting task no longer seems daunting. You have the power to take proper action for diagnosing and fixing the potential issues.

For issues like a dirty flame sensor or clogged filter, simple DIY cleaning solutions can get you back to enjoying hot water.

However, for more complex issues like a cracked combustion chamber or extensive vent blockages, don’t hesitate to seek professional help.

Don’t delay your warm and relaxing showers anymore! Use the troubleshooting guide above to diagnose the cause and get your Rheem tankless water heater back in shape!

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