21.1
Gas heating lockout; 24 VAC sensed on gas valve when it should not be
This code typically points to
- Stuck-closed gas valve relay
- Miswire
- Unwanted 24 VAC to gas valve
Next checks to confirm
- Verify unwanted voltage at gas valve
- Compare wiring to schematic
- Replace board only after field miswire is ruled out
Escalation
Strong control/relay concern if wiring is correct.
Reset behavior
Turn off power and wait 5 minutes to retry.
Safety Critical
Immediate concern