Transmission

P0871 — Transmission Fluid Pressure Sensor/Switch C Range/Performance

Severity: medium Drive with care, to a shop

What P0871 means

The transmission control module (or PCM) monitors hydraulic pressure in the valve body using pressure sensor or switch C. If its signal doesn't match the commanded gear or operating state, P0871 is stored. The cause can be a faulty sensor or a real hydraulic pressure problem.

Symptoms

  • Harsh or jerky shifts
  • Transmission warning light or Check Engine light
  • Limp mode with a fixed gear
  • Slipping or delayed engagement
  • Erratic torque converter lockup

Possible causes

  1. Failed pressure sensor/switch C
  2. Low or degraded transmission fluid
  3. Actual low pressure from a worn pump or clogged filter
  4. Valve body or solenoid fault
  5. Damaged wiring, open circuit or short to ground
  6. Corroded or loose connector
  7. Faulty TCM or PCM

How to diagnose

  1. Scan all codes and freeze-frame data and look for other transmission codes
  2. Check fluid level, color and smell, following the manufacturer's procedure
  3. Inspect the sensor harness and connector for oil, corrosion and chafing
  4. Test sensor power, ground and signal with a multimeter against the wiring diagram
  5. Compare scan-tool pressure readings to expected values in different gears
  6. Measure actual line pressure with a gauge
  7. If pressure is normal, test or replace the sensor, then evaluate the valve body
  8. Clear the code and road test to confirm the repair

Common fixes

  • Fluid and filter service
  • Replace pressure sensor/switch C
  • Repair wiring or the connector
  • Repair or replace the valve body or solenoids
  • Transmission repair for low pressure (pump, seals)
  • Software update or module replacement in rare cases

Questions and answers

What does P0871 mean?

The control module finds the signal from transmission fluid pressure sensor/switch C implausible. It could be the sensor, its wiring or a real pressure problem in the transmission.

Can I drive with P0871?

Only briefly and carefully, until it can be checked. The transmission may shift harshly or go into limp mode, and low pressure can damage it fast.

Will replacing the sensor fix it?

Not always. Check the fluid, wiring and actual pressure first, or a new sensor may not solve the problem.

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.