Usually located in the burner compartment of a contemporary gas furnace, the inducer blower induces a draft in the furnace prior to start-up to facilitate burner ignition. When the furnace start-up sequence is initiated by a signal from the thermostat, the microprocessor runs the inducer motor briefly to generate air flow. The resultant differential in the inducer pressure tube opens a pressure switch signaling that the inducer is operative and developing adequate draft. The microprocessor then lights the burner. If the pressure sensor switch does not sense operation of the inducer motor within 10 seconds, however, the processor aborts the start-up sequence and the burner does not light. All modern microprocessor-controlled furnaces by manufacturers such as Carrier include a self-diagnostic procedure to test the function of the inducer motor.
Remove the access panel covering the control board inside of the main blower cabinet. Place a piece of tape over the safety disconnect switch under the access panel cover to hold the switch down.
Locate the numbered test switches and the LED lights on the controller board.
Move test switch SW-1 into the "Off" position. Move test switch SW-6 into the "On" position to initiate the component diagnostics.
Allow the microprocessor to complete the diagnostic procedure for the inducer motor. This includes powering up the inducer motor, testing operation at low speed for 20 seconds, testing operation at high speed for 20 seconds then powering down.
Read the fault codes flashed by the LED lights. Fault code 11 means the inducer motor is functioning properly. Fault code 42 means "Inducer outside valid speed range." This indicates that 10 seconds have elapsed and the inducer motor has not started or the inducer motor has started but is not operating at expected r.p.m.
Check for a cracked or plugged inducer pressure tube leading to the pressure switch.
Check the condensation drain line for blockages and the inducer pressure tube for water. Excessive condensation may back up into the inducer pressure tube and cause the pressure switch to remain closed.
Clear any blockage in the condensate drain line and drain any water out of the inducer pressure tube. Repeat the self-diagnostic test by turning test switch SW-6 to the "Off" position then back "On" again.
Contact authorized service personnel if Fault Code 42 recurs.