Goodman GMPN 120-5 Gas Furnace-------------Turn furnace on and exhaust vent motor comes on for milliseconds to begin to turn (only a couple degrees of rotation). I can hear a hum, then, which may not to be coming from motor---not apparent to touch. With a Digital Voltmeter on input (Black to White) leads, I see some power coming on momentarily but immediately falling to zero---can't read meter to see how high the voltage is since that input power turns off so fast. I suspect that the exhaust motor is not the problem---it turns very easily with a screwdriver touch to the motor coolant fan blade. I have disconnected output hose to outside vent in my troubleshooting. What could cause this problem?

If your inducer motor is dropping out like that, it sounds like it is not proving flue. Check for restriction in exhaust pipe of furnace. Check flue switch. I hope this was helpful.