Transmission

P0842 — Transmission Fluid Pressure Sensor/Switch A Circuit Low

Severity: medium Drive with care, to a shop

What P0842 means

The transmission control module (or PCM) constantly monitors the signal from the transmission fluid pressure sensor. The voltage normally changes within a set range as hydraulic pressure changes. P0842 sets when the signal falls below the expected minimum, usually because of a short to ground, a lost sensor supply, or a failed sensor.

Symptoms

  • Transmission warning light or check engine light on
  • Limp mode with the gearbox stuck in one gear
  • Slipping during acceleration
  • Harsh or delayed shifts
  • Hard engagement when shifting into Drive or Reverse
  • Reduced fuel economy

Possible causes

  1. Failed transmission fluid pressure sensor
  2. Signal wire shorted to ground
  3. Damaged or chafed wiring harness
  4. Corroded or contaminated connector terminals
  5. Open circuit or missing sensor reference voltage
  6. Faulty transmission control module or PCM
  7. Low or contaminated transmission fluid

How to diagnose

  1. Scan all codes and freeze-frame data, note them, then clear the code to see if it returns
  2. Check the transmission fluid level and condition per the manufacturer's procedure
  3. Inspect the sensor harness and connector for chafing, fluid, moisture and corrosion
  4. Measure the sensor reference voltage and signal with a multimeter using the vehicle wiring diagram
  5. Test the signal wire for a short to ground with the connector unplugged
  6. Compare scan tool pressure readings to actual pressure with a gauge if needed
  7. Replace the sensor if wiring checks out or its readings are out of spec
  8. Test the control module and its power and grounds only after ruling out everything else

Common fixes

  • Replace the transmission fluid pressure sensor
  • Repair or replace the damaged wiring
  • Clean connector terminals or replace the connector if corroded
  • Change contaminated fluid and filter
  • Reprogram or replace the control module if confirmed faulty

Questions and answers

What does P0842 mean?

It means the signal from transmission fluid pressure sensor A is lower than expected. The cause is usually a short to ground, a poor connection, or a bad sensor.

Can I drive with P0842?

For a short distance, yes, but the transmission often goes into limp mode and may slip. Driving for long can speed up transmission wear, so diagnose it soon.

Can I just clear the P0842 code?

Clearing it only turns the light off temporarily. If the wiring or sensor is at fault, the code will return until the problem is repaired.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.