Code A4 = Flame failure during operation. The burner lit but flame dropped out before the draw ended. Distinct from Code A1 (no ignition).
Diagnostic sequence
1. Flame rod cleaning (#1 cause)
Flame rod accumulates oxidation over months/years. Cleaning with fine sandpaper resolves 60% of chronic A4 cases. See flame rod.
2. Vent restriction
Birds nest, ice, undersized vent. Restricted exhaust pushes flame characteristics outside the detection envelope.
3. Condensate blockage (condensing models)
Blocked condensate trap disrupts flame detection. Often paired with Code F2.
4. Gas pressure drop under load
Code A4 often appears when another gas appliance fires mid-draw, causing pressure drop.
5. Heat exchanger scale
Heavy scale changes burner dynamics. Run a descale.
Reset procedure
- Clean the flame rod
- Power cycle the unit
- Monitor for 24 hours
Bottom line
Flame rod cleaning resolves most A4 cases on units 5+ years old. On newer units, check vent and condensate first.