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Hums but will not start

What is going on

Possible causes and checks for: hums but will not start.

Likely causes

  • Failed start capacitor
  • Centrifugal switch contacts dirty/pitted/burnt or spring deformed
  • Failed start relay

Check in this order

  1. Replace capacitor (same µF)
  2. Replace centrifugal switch contacts (don't file — lasts hours)
  3. Replace relay. Electrical faults in windings (open/short/ground) → replace motor

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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.