Immediate safety screen
Leave the area and follow evidence-backed emergency guidance if gas odor or another immediate hazard is present.
Observations that narrow the path
- Does a diagnostic code appear before shutdown?
- Does shutdown occur before or after the indoor blower starts?
Candidate fault groups
Safety-first observation branches; not probability.
- Airflow or limit-circuit condition
- Ignition-proving condition
- Pressure-switch or venting condition
Identify the exact model, controller emitter, and displayed code before using diagnostic guidance.