Internal control fault

This code typically points to

Unspecified internal fault
Welded gas-valve relay / unexpected voltage at gas-valve contacts that does not clear by cycling inducer

Next checks to confirm

Verify fault is not external wiring/output issue
If control senses internal fault or welded-relay condition, replace control after confirming root cause

Escalation

Hard lockout; cannot be reset by cycling heat call

Reset behavior

Requires power cycle after original fault is corrected; heat-call cycling alone will not reset.

Safety Critical

Immediate concern