Code 12 = Flame failure during operation. The burner lit but flame dropped out before the draw ended. Distinct from Code 11 (no ignition at all).
Diagnostic sequence
1. Flame rod cleaning (#1 cause)
Flame rod accumulates oxidation over months/years. Unit lights normally; signal degrades during operation; controller misreads as flame lost; shuts down. Cleaning resolves 60% of chronic Code 12 cases. See flame rod.
2. Vent restriction
Birds nest in exhaust termination, ice over winter, undersized vent. Restricted exhaust pushes flame characteristics outside detection envelope.
3. Condensate blockage (condensing models)
Blocked condensate trap backs up into burner area, disrupts flame detection. Often paired with Code 25. Clear the drain.
4. Gas pressure drop under load
Code 12 often appears when another gas appliance fires mid-draw (furnace cycle, range burner). Pressure drops below 4" WC minimum; flame goes weak; sensor misreads.
5. Heat exchanger scale
Heavy scale changes burner dynamics. Run a descale.
6. Combustion fan
Failing fan motor changes combustion air. Often paired with Code 29.
Reset procedure
- Clean the flame rod (15 minutes)
- Power cycle the unit
- Monitor for 24 hours under typical draws
- If code recurs, escalate to vent / condensate / gas pressure
Bottom line
Flame rod cleaning resolves most Code 12 cases on units 5+ years old. On newer units, check vent and condensate first.