P0842
Transmission Fluid Pressure Sensor/Switch A Circuit Low.
- Generic
- Powertrain
- Generic code
- Medium severity
How this code is built
- P
- SystemPowertrain
- 0
- Defined bySAE, the same on every car
- 8
- AreaTransmission
- 42
- FaultThe specific fault in that area
What it means
P0842 is the generic code for a low signal from the transmission fluid pressure sensor or switch 'A' circuit. The transmission control unit monitors the sensor's voltage, and it has seen a signal below the expected range, which points to a short to earth, an open circuit, a failed sensor or a supply fault. The exact sensor location and the signal range depend on the make, model and transmission.
Symptoms
- Gearbox warning light or malfunction indicator lamp on
- Transmission stuck in a single gear (limp mode)
- Harsh, delayed or flared gear changes
- Slipping or loss of drive in some gears
- Gearbox temperature or pressure related message on the dash (varies by model)
Possible causes
- Faulty transmission fluid pressure sensor or switchlikely
- Damaged, chafed or shorted wiring to the sensor, especially where the loom runs near the gearboxlikely
- Corroded, loose or fluid-contaminated connector at the sensor or control unitlikely
- Low or degraded transmission fluid causing genuinely low pressure or erratic readingspossible
- Internal hydraulic problem such as a worn pump, blocked filter or valve body fault causing real low pressurepossible
- Poor sensor supply or earth, or a fault in the circuit feeding the sensorpossible
- Faulty transmission control unitrare
How to diagnose
- Read all stored and pending codes and the freeze frame data, noting any other gearbox or power supply faults
- Check the transmission fluid level and condition using the manufacturer's procedure, as many modern gearboxes need a set temperature and method
- Visually inspect the sensor, its wiring and connector for damage, leaks, corrosion and bent or pushed-back pins
- Check the battery condition and charging voltage, as low system voltage can upset sensor readings
- Using the wiring diagram for the vehicle, test the sensor supply, signal and earth for continuity, short to earth and short to supply
- Monitor live sensor data and compare it with the specified values while the gearbox is warm and in different gears
- Check the line pressure with a suitable gauge if the manufacturer provides a test procedure and the sensor appears to be good
- Clear the code and road test to see if it returns, and confirm that any repair has worked
How to fix
- Replace the transmission fluid pressure sensor or switch if it reads out of range or fails testing
- Repair or replace damaged wiring and clean or renew corroded or damaged connector terminals
- Correct the fluid level or change the fluid and filter to the manufacturer's specification if it is low or degraded
- Rectify any supply or earth fault found in the sensor circuit
- Repair the gearbox hydraulics, such as the pump or valve body, if measured pressure is genuinely low
- Repair or replace the transmission control unit only when all other causes are ruled out and it is confirmed faulty
Can I keep driving?
Medium severity
The car may be driveable in limp mode, but gearbox performance and protection are reduced. Have it checked soon, because genuinely low fluid pressure can quickly cause serious internal damage.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.