Rheem error code 12 means the flame signal is weak. The furnace may still run, but the sensor, wiring, grounding, or burner flame should be checked before the condition causes shutdown.
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Error code 12 means the furnace is detecting a weak or marginal flame-sense current. Heating may continue, but the condition can progress to flame loss or failed ignition.
Furnace may continue heating while displaying code 12; intermittent burner shutdown; occasional ignition retries; heating may later fail with code 11 or 13.
Dirty or oxidized flame-sensor rod; loose, shorted, or open flame-sensor wiring; improperly mounted sensor; poor equipment grounding; damaged flame-sensor porcelain; weak or unstable burner flame.
1. Turn off furnace power. 2. Locate and inspect the flame sensor for visible contamination, damage, or a loose wire. 3. Review the Flame Sensor Troubleshooting Guide at /flame-sensor-troubleshooting/. 4. Make sure the furnace access panels are properly installed before restoring power. 5. If the code remains, a qualified technician should measure flame-sense current, verify grounding, inspect burner flame quality, and test the sensor circuit.
Flame sensor; flame-sensor wire; burner assembly; grounding connections; integrated furnace control.
Recommended if the code returns after safe sensor inspection or if the furnace begins losing flame or entering ignition lockout.
Turn off electrical power before accessing the flame sensor. Do not touch burners, gas piping, or live control components.
Learn more about the furnace components and operating conditions commonly associated with diagnostic error codes: