Upstairs unit operating in heat; yesterday the circuit board was not sending 24 volts to the gas valve until pushing in all the wire connections to the molex connector. More than likely the unit did the same thing intermittently.Replaced circuit board and rechecked every low and high voltage connection. Cleaned flame sensor, cleared draft port, checked gas pressure and temperature rise. all ok at this time. the duct work to this unit should be replaced soon; in poor condition.