P0871

What it means

P0871 is a generic code meaning the transmission control unit has detected that the signal from transmission fluid pressure sensor/switch 'C' is out of its expected range or does not match the pressure it expects for the current gear and commanded state. The circuit is not necessarily open or shorted, but the reading is implausible. Which hydraulic circuit sensor 'C' monitors depends on the make, model and gearbox.

Symptoms

• Gearbox warning light or malfunction indicator lamp on
• Harsh, delayed or flared gear changes
• Gearbox stuck in one gear (limp mode)
• Slipping or loss of drive in certain gears
• Torque converter lock-up not working, with poor economy
• Gearbox runs hot or shifts erratically when warm

Possible causes

• Low, degraded or contaminated transmission fluid, or incorrect fluid levellikely
• Faulty pressure sensor/switch C giving a drifting or implausible signallikely
• Damaged wiring, corroded connector or poor earth or supply to the sensor, or fluid ingress into the harnesslikely
• Blocked internal filter or restricted valve body passage affecting pressure to that circuitpossible
• Sticking or worn solenoid or valve body fault causing real pressure to differ from the commanded valuepossible
• Worn pump or internal seal leakage causing genuinely low pressurerare
• Fault within the transmission control unit or outdated software known to affect this code on some modelsrare

How to diagnose

1. Read all stored codes and freeze frame data, noting gear, fluid temperature and speed when the fault set, and check for related solenoid or ratio codes
2. Check the fluid level and condition using the correct procedure for the gearbox, looking for burnt smell, debris or foaming
3. Inspect the sensor wiring and connector for chafing, corrosion, oil contamination and bent or loose pins
4. Check the manufacturer's data for sensor C location and expected readings, as this varies by model and gearbox
5. Watch live pressure sensor or switch data against commanded gear and compare with specification
6. Check sensor supply voltage, signal and earth with a multimeter, and test for wiring resistance faults
7. Check for any software updates or technical bulletins relevant to the vehicle
8. If wiring and sensor are sound, check line pressure and look into valve body, filter and solenoid condition as the manufacturer directs

How to fix

• Correct the fluid level or replace degraded fluid and filter with the specified type
• Repair or replace damaged wiring and clean or replace corroded connectors
• Replace the faulty pressure sensor or switch and clear the code
• Clean or replace the valve body, solenoids or internal filter if restriction or sticking is found
• Repair or rebuild the pump or internal seals if pressure is proven low
• Update or replace the control unit software only where an outdated calibration is a documented cause

Can I keep driving?

Nearby codes

• P0870GenericTransmission Fluid Pressure Sensor/Switch C Circuit
• P0872GenericTransmission Fluid Pressure Sensor/Switch C Circuit Low