E02 = flame failure during operation. The unit ignited successfully but lost flame detection mid-cycle. Distinct from E01 (which is failure to ignite at all).
Common causes
1. Vent obstruction or back-pressure
- Blocked or partially blocked PVC vent termination — snow, debris, nests
- Condensate trap clogged → back-pressure → flame instability → E02
- Vent equivalent length exceeded due to additional elbows in retrofit
2. Gas pressure drop under load
- Gas supply adequate at low fire but drops under high fire
- Common when multiple gas appliances run simultaneously (range + tankless + dryer)
- Undersized gas line shows up under load
- LP tank cold or low
3. Flame sensor degradation
- Marginal flame sensor reads at the edge of detection threshold
- Carbon deposits or oxidation cause intermittent loss of signal
- Clean or replace flame sensor
4. Combustion air supply
- Air intake blocked or partially restricted
- Combustion air mix drifts; flame becomes unstable
5. Heat exchanger scale insulating flame area
- Heavy scale changes heat transfer dynamics
- Flame becomes unstable on scaled tubes
- Descale
Diagnostic sequence
- Inspect vent termination — clear obstructions, verify slope
- Check condensate trap — clean if clogged
- Clean flame sensor (sandpaper)
- Test gas pressure under load with manometer
- Verify intake termination is clear
- Descale heat exchanger if not done in past year
Reset
- Address suspected cause
- Press Reset on control display
- Open hot tap; monitor for recurrence
Bottom line
E02 mid-operation flame loss differs from E01 ignition failure. Most cases: vent issue or marginal flame sensor. Annual service (descaling, sensor cleaning, vent inspection) prevents most E02 events.