21
Gas heating lockout
This code typically points to
- Stuck-closed main gas valve relay on control
- Miswire/short to gas-valve wiring
- Flame-sense loss after burner-on time (in some lockout scenarios)
Next checks to confirm
- Verify gas-valve wiring is correct (blue to M / green-yellow to C)
- Isolate miswire/short vs relay failure using OEM steps
- If lockout occurs only when pressure switch makes, suspect control
Escalation
If wiring is correct and code still flashes under proven conditions, control board becomes likely.
Reset behavior
Does NOT auto-reset.
Safety Critical
Immediate concern