Troubleshooting, Service, and Safety
Troubleshooting, Service, and Safety
This domain tests systematic troubleshooting and safety judgment. Start with the complaint, then verify the call for heat, the sequence of operation, and the safety devices. A furnace that locks out is telling you which proof failed; read the fault code before replacing parts.
Common failures:
- No inducer: check the board, capacitor, and motor.
- Pressure switch not closing: blocked flue, water in the tubing, wrong switch, or weak inducer.
- Igniter glows but no flame: gas supply, valve, orifice, or igniter position.
- Burner lights and drops out: flame sensor, ground, polarity, or gas pressure.
- Limit trips: restricted airflow, dirty filter, bad blower, or oversized input.
- Rollout trips: blocked flue, cracked heat exchanger, or improper draft.
Safety
Treat CO as the primary hazard. A combustion analyzer and a draft gauge are more reliable than flame color. If you find spillage, flame rollout, or rising air-free CO, shut off the appliance and red-tag it. Never vent an appliance into a space, never operate with the flue disconnected, and never bypass a safety to get heat on.
Worked example
A furnace runs with a yellow, lazy flame and the rollout switch trips. The flue is partially blocked by a bird nest. The blocked flue reduces draft, products spill at the burners, and the rollout switch opens. Clearing the flue and verifying draft with a manometer and analyzer restores safe operation. Replacing the rollout switch alone would have left a CO hazard.
Traps
Do not reset a manual rollout switch without finding the cause. Do not assume a dirty flame sensor is the only reason for short cycling. Do not ignore return-air restrictions or an overfired burner. Do not use a match or flame to test for gas leaks.