Transmission

P0840 — Transmission Fluid Pressure Sensor/Switch A Circuit Malfunction

Severity: medium Drive with care, to a shop

What P0840 means

The transmission control module (TCM, or the PCM on some vehicles) monitors hydraulic line pressure through pressure sensor or switch A. P0840 sets when the signal is outside its expected range or does not match what the transmission is doing. Without reliable pressure data, the module cannot control shift timing and clutch apply pressure accurately.

Symptoms

  • Check Engine or transmission warning light on
  • Harsh, delayed or flaring shifts
  • Transmission in limp mode, stuck in one gear
  • No shifts or some gears unavailable
  • Slipping under acceleration
  • Reduced fuel economy

Possible causes

  1. Failed transmission fluid pressure sensor or switch
  2. Damaged wiring or corroded sensor connector
  3. Wiring fault in the valve body connector
  4. Low, dirty or degraded transmission fluid
  5. Clogged passages or a faulty valve body
  6. TCM/PCM fault

How to diagnose

  1. Scan all codes and freeze-frame data, noting any other transmission codes
  2. Check fluid level and condition: color, smell, metal debris
  3. Inspect the sensor and valve body connectors and harness for corrosion, fluid and chafing
  4. Test power, reference voltage and ground at the sensor connector with a multimeter
  5. Check the signal wire to the TCM for opens and shorts
  6. Compare live pressure sensor data with expected values at different operating conditions
  7. Test the sensor against factory specifications
  8. If sensor and wiring are good, evaluate the valve body and TCM

Common fixes

  • Correct fluid level or replace fluid and filter if contaminated
  • Replace the pressure sensor or switch
  • Repair wiring or the connector and clean the terminals
  • Repair or replace the valve body
  • Replace or reprogram the TCM once confirmed faulty

Questions and answers

What does P0840 mean?

It means the control module detected a problem in the circuit of transmission fluid pressure sensor/switch A. The signal is out of range, so the module cannot judge fluid pressure correctly.

Can I drive with P0840?

Only briefly and carefully, to get to a shop. The transmission may be in limp mode, and harsh shifts increase wear.

What most often causes P0840?

Most often it is the pressure sensor itself, damaged wiring, or a bad connector at the valve body. Fluid problems, the valve body or the TCM are less common causes.

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.