Troubleshooting, Diagnosis, and Service Procedures
What the exam tests
Systematic diagnosis, instrument use, safety, common failure patterns, and service documentation.
Core rules
- Verify the complaint, thermostat call, power, and fault codes before disassembly. Observe the sequence; the point of failure narrows the circuit.
- Use a manometer for gas pressure, a multimeter for voltage/continuity/microamps, and a combustion analyzer for CO, CO2, O2, and draft. Confirm instrument calibration and zero in fresh air.
- Common faults: dirty flame sensor (clean with steel wool, not sandpaper), failed HSI, blocked pressure-switch tubing, clogged condensate trap, dirty filter, failed inducer, and gas valve not opening.
- Never bypass a limit, rollout, or pressure switch except for manufacturer-approved testing. If CO is elevated or a heat exchanger is suspect, red-tag and shut down.
- Document supply/return temperatures, static pressure, manifold pressure, flame current, and CO readings before and after repair.
Worked example
A furnace locks out after three trials. The igniter glows and the valve clicks, but no flame. Manifold pressure is 0 in. w.c. during trial. Gas supply at the inlet is 7 in. w.c., so supply is present. The valve receives 24 V but does not open; the valve operator or board output is faulty. If 24 V is absent at the valve, trace the board and series safeties.
Traps
Do not replace parts by guesswork; verify voltage and pressure at the component. Do not overlook a nearly empty propane tank or a shut meter valve. Do not use a combustible-gas detector as the only leak test. Do not leave a unit operating with a known CO leak.