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No heat (ignition stage)

What is going on

Generic sequence

Likely causes

  • No 120 V/24 V
  • Open door interlock
  • Open limit/rollout/pressure switch
  • Inducer not running
  • Pressure switch not closing (blocked vent/condensate)
  • Igniter not glowing (cracked/open HSI
  • No output from module
  • Bad transformer)
  • No spark (bad lead
  • Gap)
  • Gas valve not opening
  • Flame not proven (dirty/cracked flame rod
  • Grounding
  • Polarity)
  • Lockout

Check in this order

  1. Observe how far the start-up sequence gets
  2. Read board flash code
  3. Verify 120 V at igniter then at module output
  4. Ohm the igniter power-off
  5. Confirm inducer runs and pressure switch closes
  6. Check limits with meter
  7. Manometer at manifold
  8. ΜA flame signal. Service scenarios: no voltage at igniter and none at module output with good igniter resistance → replace module (41A)
  9. Burners light then drop out, film on flame rod → clean rod (41B)
  10. Inducer running, pressure switch closed, igniter not glowing, igniter ohms open → replace igniter (41C)

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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.