Rheem R801SA furnace code 12: Low flame signal
Professional repair
What does code 12 mean on a Rheem R801SA (801S downflow) furnace display? Code 12 means the control is sensing a weak flame signal. The code reports the signal level only; it does not by itself prove the flame sensor is dirty.
This condition involves the gas or combustion side. If you smell gas or a CO alarm sounds: leave, then call your gas utility. Diagnostic info here is reference — combustion repairs are for licensed technicians. All six emergency signs →
A technician measures flame current, inspects the flame and sensor, and checks grounding, polarity and gas pressure before cleaning or replacing anything. Sensor removal is not a homeowner task.
Possible causes
- Contaminated flame sensor rod reducing the flame signal TECH
- Poor grounding or reversed polarity at the furnace TECH
- Gas pressure affecting how the flame contacts the sensor TECH
Parts for this code
No model-verified part number is listed here. Have the fault diagnosed and the correct part matched to your furnace before ordering.
Questions about this code
What does code 12 mean on a Rheem R801SA (801S downflow) furnace display?
Code 12 means the control is sensing a weak flame signal. The code reports the signal level only; it does not by itself prove the flame sensor is dirty.
How do I fix Rheem R801SA furnace code 12?
Schedule service soon rather than waiting for a lockout. Note whether heat cycles normally or stops early so the technician can compare. The fix depends on the measured flame current and what is pulling it down.
How much does it cost to fix Rheem R801SA furnace code 12?
The bill reflects diagnostic time to measure flame current and find the cause. A sensor service, wiring or ground correction, or gas adjustment each involves different labor.
Applies to
R801SA / 801S downflow single-stage 80% furnace with PSC motor
Different Rheem generations use different code maps. If your series isn't listed, don't assume this meaning — check your blower-door label.
Verified against Installation Instructions for Downflow Single Stage Gas Furnaces, p.34
Verified against Rheem 801S Series Installation Instructions — OEM143-14 mirror, p.34
Last verified 2026-10-07 · page updated 2026-10-07 · how we verify
Related Rheem furnace codes
These codes use the same rheem 801s downflow single-stage control (manual 92-24161-143-14) system.