Low transmission fluid level
Dirty transmission fluid
Faulty shift solenoid ‘D’ valve
Shift solenoid ‘D’ valve harness or connectors
Shift solenoid ‘D’ valve circuit is open or shorted
Engine Light ON (or Service Engine Soon Warning Light)
Transmission will not shift gears
Shift solenoid valve ‘D’ is turned ON or OFF by the Transmission Control Module (TCM) in response to signals sent from the park/neutral position (PNP) switch, vehicle speed and Engine Control Module (ECM) (throttle opening). Gears will then be shifted to the optimum position.