Check ECM high voltage power circuits

What is going on

This is the test for line voltage getting to the ECM blower motor through the 5-circuit power connector. Straightforward once you know the pin pairs. You will have power on while you meter, so keep your hands clear and use the right meter range. A motor can look dead when it simply isn't being fed.

Likely causes

• No supply voltage reaching the furnace
• A bad connection or break in the 5-circuit harness feeding the motor

Check in this order

1. Disconnect power, then unplug the 5-circuit power connector from the ECM motor.
2. Turn power on and measure between pins 4 and 5. It should equal the furnace supply voltage.
3. Measure between pins 4 and 3. It should be about zero volts.
4. Measure between pins 5 and 3. It should equal the furnace supply voltage.
5. If there is no voltage, check the furnace supply voltage (see section S-1).
6. Disconnect power and reconnect the 5-circuit harness.

More furnace guides

• Airflow lower than demanded, furnace runs on low stage when high stage is expected
• Alternate combustion air opening (upflow or horizontal)
• Alternate vent/flue location (upflow or horizontal left)
• Annual inspection checklist
• Auxiliary limit control (location and purpose)
• Auxiliary limit switch open (blower compartment)