Code 012 (or E012) = Flame failure during operation. Burner lit successfully but flame dropped out before the draw ended. Distinct from Code 003 (no ignition at all).
Diagnostic sequence
1. Flame rod cleaning (the #1 cause)
Flame rod accumulates oxidation over months/years. Unit lights, signal degrades during operation, controller misreads as flame lost, shuts down. Cleaning resolves 60% of chronic 012 cases. See flame rod page.
2. Condensate blockage (condensing models)
Blocked condensate trap backs up into the burner area, disrupts flame detection, throws 012. Often paired with Code 041. Clear the drain.
3. Vent restriction
Birds nest in exhaust termination, ice over winter, undersized vent. Restricted exhaust pushes flame characteristics outside the detection envelope. Inspect the termination.
4. Gas pressure drop under load
Code 012 often appears when another gas appliance fires mid-draw (furnace cycle, range burner). Gas pressure drops below 4" WC minimum, flame goes weak, sensor misreads, shutdown. Have utility check incoming pressure or upsize gas line.
5. Heat exchanger scale
Heavy scale changes burner dynamics. Run a descale.
6. Combustion fan (less common)
Failing fan motor changes combustion air, flame characteristic shifts. Often paired with Code 030.
What does NOT cause Code 012
- Igniter — only relevant for initial ignition (Code 003)
- PCB / control board — rare standalone cause
Reset procedure
- Clean flame rod (15 minutes)
- Power cycle the unit
- Monitor for 24 hours under typical draws
- If code recurs, escalate to condensate / gas pressure / vent inspection
Bottom line
Flame rod cleaning resolves most 012 cases on units 5+ years old. On newer units, check vent and condensate first. See troubleshooting hub for symptom-based diagnostic.