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All brands · Furnace troubleshooting

Does not start

What is going on

Possible causes and checks for: does not start.

Likely causes

  • Relay won't close/dirty contacts
  • Safety lockout open
  • Bad relay coil
  • Low voltage
  • Open high limit
  • Broken/loose wires
  • Relay transformer open
  • Thermostat open (dirty contacts)
  • Stack switch open
  • Heat-sensing contacts out of place
  • Motor overload open

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Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.