33.1
Switch opened in the main limit / main limit branch open
This code typically points to
- Airflow restriction
- Blower problem
- Open limit
- Dirty filter, oversized temperature rise, closed registers, weak motor/capacitor on non-ECM applications
Next checks to confirm
- Verify airflow/static and filter
- Verify blower operation and heat rise
- Inspect limit circuit
Escalation
Repeated main-limit opening after airflow correction suggests switch or HX/combustion concern.
Reset behavior
Transitions into 13.1 if lockout criteria are met.
Urgent
Fix now