P0840Powertrain
Powertrain · SAE
Transmission Fluid Pressure Sensor/Switch A Circuit
Meaning
The transmission control module has detected a fault in the transmission fluid pressure sensor/switch A circuit. The sensor signal is outside expected specification parameters.
Common causes
- Faulty or out-of-calibration ATF pressure sensor/switch A
- Short to battery or short to ground in the sensor signal wire
- Open circuit (broken wire or disconnected connector) in the sensor circuit
- Oxidised or corroded connectors on the transmission housing
- Actually low hydraulic pressure in circuit A triggering the sensor
- Internal TCM fault on the reading channel
Symptoms
- Check engine or transmission warning light on
- Abnormal gear shifts or transmission in limp mode
- Sudden changes in shift quality
- Sometimes no obvious driving symptoms
Severity
Moderate to high: an incorrect pressure signal can cause the TCM to operate the transmission non-optimally, risking clutch damage.
What to do
- Check ATF level and condition
- Measure pressure sensor A voltage and resistance with a multimeter
- Check wiring to the sensor for shorts or open circuits
- Measure actual hydraulic pressure in circuit A with a gauge and compare with the sensor signal
- Replace pressure sensor/switch A if faulty