Rheem Tankless Error Code 12: Flame Loss

The Short Answer: On Rheem gas tankless heaters, code 12 is a flame-loss fault: the burner lost its flame during operation. Rheem’s owner manual for RTG models links it to an interrupted gas supply, or a propane tank running low. Check that gas service is on and the propane tank has fuel. If the code returns with a good supply, a technician needs to check gas pressure, venting, and the flame sensor.

What Rheem’s documents say

Rheem’s Use & Care manual for RTG non-condensing tankless models (opens in a new tab) lists two owner-level causes for code 12:

On the newer condensing IKONIC models, Rheem’s service guide (opens in a new tab) uses A012, “Unit Detected Flame and Lost It,” posted after repeated flame losses in a row. The guide’s checks for technicians include gas pressure, the gas and air filters, venting, the flame rod, and the gas valves.

Other Rheem tankless models and older generations may word this differently. Check the manual for the model on your rating plate.

Why a burner loses its flame

• Gas supply that sags under load. A tankless burner needs a lot of gas at full fire. An undersized line, a small meter, or other gas appliances running at the same time can starve it.
• Propane problems. A low tank, or a regulator struggling in very cold weather.
• Venting and air. Blocked terminations, wind blowing into an outdoor unit, or a clogged air filter on models that have one.
• Flame sensing. A dirty or worn flame rod can lose track of a flame that is actually there.

What you can check safely

1. Make sure other gas appliances work. If none do, call your gas utility.
2. Check the propane level and that the tank valve is fully open.
3. Confirm the gas valve at the heater is fully open.
4. Look at the vent terminations or the outdoor unit for snow, ice, debris, or nests.
5. Note whether the code appears with high demand, in cold weather, or randomly.
6. Reset once by turning the unit off and on. If 12 returns, leave it off and call for service.

What the technician will check

A technician will measure gas pressure at the heater while it runs at full fire, to see whether the supply drops. They inspect the vent and air intake, clean or replace the flame rod, and check the gas valve and combustion fan. On condensing models they also check the condensate drain, since a backed-up drain can affect combustion.

Repair or replace

Code 12 usually leads to a repair: a gas supply correction, a propane regulator, a flame rod, or a venting fix. Replacement isn’t normally the answer to a flame-loss code on its own.

For a burner that never lights, see Rheem tankless flashing 11. For broader issues, see Rheem tankless water heater troubleshooting.

Frequently Asked Questions

Q: My propane tank isn’t empty. Why do I still get code 12? A: A partly full tank can still fail to deliver enough gas in very cold weather, and a regulator can freeze or fail. If the tank is reasonably full, have your propane supplier or a technician check the regulator and pressure.

Q: Is code 12 the same as code 11? A: No. Code 11 relates to ignition: the burner didn’t light. Code 12 means it was running and lost the flame.