Inducer Motor Diagnostic

Testing the inducer motor

Use this procedure to diagnose if the inducer motor is the source of your furnace problem — after you’ve done basic troubleshooting.

Tools needed

• multimeter with setting to measure AC voltage or induction current
• multimeter leads with clamps (helpful but not required)

Testing procedure

1. Remove the upper furnace cabinet cover.
2. Locate the inducer motor and follow the electrical wires to the connector on the control board. Disconnect it.
3. Set your multimeter to measure AC voltage.
4. Insert one probe of the multimeter into the pin on the connector that corresponds with the white wire.
5. Insert the other probe into the pin on the connector that corresponds to the black wire (if its a three-wire connector, choose either of the other non-white pins).
6. Initiate call for heat and watch the multimeter. Note the voltage measurements.
   • For three-wire connectors: if you did not measure any voltage at all, leave one probe in contact with the pin for the white wire, but switch the other probe to the third pin. Reset the furnace power and repeat this step.
7. Turn off the power to the furnace.
8. Reconnect the inducer motor wire.

Interpreting the results

When there is a call for heat the inducer motor must turn on before the other components to prepare for ignition. If it fails to turn on or reach the speed required for proper combustion, the furnace sequence of operations will stop.

The following are possible causes for inducer motor problems:

• inducer motor has failed
• open limit or roll out switch (likely indicated by a furnace error code)
• control board relay for the inducer motor has failed, preventing power delivery
• external run capacitor has failed

Determine which of the following cases matches your result:

Measured 120V or similar

If you measured 120V or similar, the control board is providing power and the inducer motor should run and ramp up to speed when connected. If not, it has failed and needs to be replaced.

Some inducer motors have an external run capacitor. If the run capacitor fails or is operating out of spec, the inducer motor may not run or ramp up properly, causing insufficient combustion air flow and pressure switch errors. If you are seeing this behaviour, go to the run capacitor diagnostic.

No voltage measured at any time

If you did not measure voltage at any time, during the call for heat (and you are certain the inducer should otherwise be receiving power) the control board should be replaced.

Note: The inducer motor usually fails due to bearings on the motor wearing out. You will generally hear this for days or even months leading up to the eventual failure of the inducer motor.