Forced Air Furnace: No-Heat

Bad Inducer Motor Diagnostic Tree

* These diagnostic trees are primarily for training purposes and may not cover all possible scenarios. 

1) Is the inducer motor running?
2) 24 V on control board between W and C terminals?
3) 24 V on control board between R and C terminals?
Proceed to testing thermostat → Bad Thermostat Diagnostic Tree
Proceed to verify control voltage → link
4) Is safety loop open? (high limit switch, flame rollout, etc.)
5) Is pressure switch closed? (0 volts read across both sides)
Pressure switch stuck closed. Furnace will not start heating sequence until the switch is proven open.
6) 120 V to inducer motor? (Hot 120 V lead and Neutral present at molex connection)
7) Does the motor spin freely by hand?
Squirrel cage may be jammed. Remove and inspect. If clear, motor bearings are bad. Replace motor.
8) Does the inducer motor have a capacitor?
Bad motor. Ohm windings to verify (~5–30 Ω).
9) Test capacitor microfarads. Is the capacitor good or bad?
Motor is bad. Ohm windings for verification (~5–30 Ω expected).
Replace capacitor.
10) Is the furnace control board sending out 120 V?
Likely bad relay in control board.
Ohm out 120 V (hot) wire from both ends and Neutral wire from both ends.
OL or > 1 Ω requires replacing wiring.