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No heat

What is going on

Possible causes and checks for: no heat.

Likely causes

  • No main power
  • Faulty thermostat
  • Faulty limit
  • Faulty flame sensor
  • Faulty ignition control module
  • Faulty inducer motor
  • Faulty wiring harness
  • Broken/shorted igniter
  • Faulty combustion relay
  • Flame sensor out of flame / low µA
  • Open auxiliary limit

Check in this order

  1. Test voltage
  2. Test thermostat
  3. Test fan & limit control
  4. Test flame sensor
  5. Test ignition module
  6. Test motor
  7. Test wiring
  8. Test igniter
  9. Test relay
  10. Test flame sensor
  11. Push manual reset on aux limit

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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.