What It Means
Flame established but microamp flame sense signal is weak.
Rheem Furnace
Rheem Furnace Error Code 12
Troubleshooting guide for the Rheem Furnace system fault 12.
Possible Causes
- Carbon or oxide buildup on flame sensor.
- Poor burner or chassis grounding.
- Flame sensor misaligned in burner flame.
- Dirty burner or gas pressure issue.
- Loose sensor wiring or uncalibrated board.
Not sure which one it is — or if it's none of these? Try ACLogics today.
Disclaimer: This information is for general reference. Error codes can vary by model and production year. Always refer to your machine's specific manual and the information printed on the unit itself.
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Full Diagnosis
Flame was established but the microamp signal the flame sensor reports back to the control is below the minimum the control accepts as a reliably proven flame, so the control treats it as a marginal/at-risk flame condition. This is a step past 11 (no flame at all) — ignition succeeded but the sensing circuit is not confident the flame will hold — and is a different failure path from 13, where a previously strong, proven flame drops out entirely mid-cycle. Depending on model, repeated low-sense events can trigger a soft lockout the same as outright ignition failure.
All Recorded Causes
Carbon or oxide buildup coating the flame sensor rod, poor chassis/burner grounding reducing rectification current, flame sensor positioned outside the optimal part of the flame, degraded flame quality from a dirty burner or gas pressure issue, corroded or loose sensor lead, or a control board losing calibration on the flame-sense circuit. Clean and reposition the sensor and verify grounding before replacing the control.
Verify Before Replacing Parts
- Confirm the exact Rheem model family and furnaces context before acting on a generic code description.
- Walk the entire heating sequence in order instead of jumping straight to the reported code, watching for carbon or oxide buildup coating the flame sensor rod, poor chassis/burner grounding reduci….
- Verify each safety device and prove where the sequence stops under live operation, including whether clean and reposition the sensor and verify grounding before replacing the control.
- After the correction, run the system long enough to confirm the furnaces fault does not return under normal load.
Still unsure about this fault?
ACLogics walks you through Rheem Furnace diagnostics step-by-step — so you confirm the cause before swapping parts.
