Code 12 = Flame failure during operation. Distinct from Code 11 (no ignition).
Diagnostic sequence
1. Flame sensor cleaning (#1 cause)
Carbon buildup degrades signal. Cleaning resolves 60% of cases on older units.
2. Vent restriction
Birds nest, ice, undersized vent. Inspect termination.
3. Condensate blockage
Blocked condensate trap. Often paired with Code 14 or Code 25.
4. Gas pressure drop under load
Code 12 often appears when another gas appliance fires mid-draw.
5. Heat exchanger scale
Heavy scale changes burner dynamics. Run a descale.
Reset
- Clean flame sensor
- Power cycle
- Monitor for 24 hours
Bottom line
Flame sensor cleaning resolves most Code 12 cases on units 5+ years old.