5 blinks
Twinning fault
This code typically points to
- Thermostat wire on twin terminal
- Incorrect board switch setting
- Mismatched boards in twinned furnaces
- Transformers out of phase
- Thermostat/wiring issue
Next checks to confirm
- Verify twin wiring only between TWIN terminals
- Confirm both IFCs are same part number
- Verify transformers are in phase
- Remove thermostat wires and jumper R to W to isolate thermostat/wiring fault if needed
Escalation
Fault blocks correct twinned operation until configuration/wiring issue is corrected
Reset behavior
Clears when twinning wiring/configuration is corrected; normal operation resumes after fault removal.
Urgent
Fix now