HVAC Electrical Troubleshooting: Contactors, Capacitors, Relays, and Meters

The short field answer:
Good electrical troubleshooting is ordered, not random. Start with safety. Then confirm the control signal. Then see whether the device actually closes. Then see whether line voltage passes through. Then verify that the motor or compressor draws current and starts correctly. OEM fault trees repeatedly point to bad run capacitors, failed contactors, low pilot voltage, open circuits, and damaged windings as common causes behind “the unit won’t start.” (lennox.com)

Lennox fault tables explicitly connect locked-rotor and open-start/open-run problems to failed run capacitors, failed contactors, low pilot voltage, open supply wiring, and damaged windings. That is why a meter is not just “for checking power”; it is how you isolate whether the failure is in the control path, switching device, or load itself. (lennox.com)

Safety that matters:
Do not trust capacitors just because power is off. Trane literature repeatedly instructs techs to disconnect power, discharge motor start/run capacitors, and verify with an appropriate voltmeter that the capacitors are actually discharged before servicing. (Trane)