– Dirty transmission fluid
– Faulty shift solenoid ‘D’ valve
– Shift solenoid ‘D’ valve harness or connectors
– Shift solenoid ‘D’ valve circuit is open or shorted
– 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.