During a service call on a mid-efficiency furnace where the induced draft motor will not operate, what should be checked for voltage?

In a mid-efficiency furnace, when the induced draft motor fails to operate, checking the control voltage across terminals "W" and "C" at the circuit board is crucial. This step is essential because the terminals "W" and "C" are part of the thermostat circuit that sends a call for heat to the furnace control board. If there is proper control voltage present, it indicates that the furnace is receiving the necessary signal to initiate the heating cycle, which includes powering the induced draft motor.

If there is no voltage across these terminals, it suggests an issue with the signal from the thermostat or a problem further up the control circuit, preventing the furnace from proceeding with the ignition sequence, including activating the induced draft motor. This check can help diagnose whether the problem lies in the control circuitry or potentially another component, aiding in efficient troubleshooting.

In contrast, while checking the thermostat connections and the induced draft relay could be part of the overall diagnostic process, they are not as direct as verifying the control voltage at the circuit board. The gas valve is similarly not involved in the initial check for the induced draft motor's operation since it should only open after the motor has started and ignition is confirmed.