P0871

Code

Generic P — Powertrain

Transmission Fluid Pressure Sensor/Switch C Circuit Range/Performance

Brand: Generic

Type: P — Powertrain

Views: UK: 28 EN: 115 RU: 43

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Page language: EN

Causes

Faulty transmission fluid pressure sensor or pressure switch (C)
Open, shorted, or corroded wiring or connector in the sensor/switch C circuit
Poor electrical connection or terminal corrosion at connector
Low or contaminated transmission fluid (level/condition)
Internal transmission hydraulic problem (valve body, pressure regulator, blocked passage)
Faulty TCM/PCM or incorrect programming

Symptoms

Check Engine or Transmission warning light illuminated
Harsh, delayed, or erratic shifting
Transmission may go into limp-home mode (reduced gear availability)
Possible inability to shift or slipping under load
Stored P0871 code, may be accompanied by related transmission codes

What to check

Retrieve all stored codes and freeze frame data with a scan tool; note related transmission codes
Inspect transmission fluid level and condition (color, smell, contamination)
Visually inspect wiring and connector for sensor/switch C for damage, corrosion, or looseness
Backprobe sensor connector and monitor live data while cranking/running to confirm signal behavior
Check for continuity, short to ground, and short to battery on the sensor circuit (with ignition off)
Compare sensor signal (voltage/resistance) to manufacturer specification or typical expected range

Signal parameters

Typical variable pressure sensor: approx. 0.5–4.5 V output proportional to pressure (varies by vehicle)
Typical pressure switch: open/closed digital state (0 V or near-battery voltage when switched)
Open circuit: no signal/OL; short to ground: ~0 V; short to battery: near B+ (~12 V)
Expected circuit resistance to ground when disconnected depends on sensor type; consult factory spec

Diagnostic algorithm

Connect an OBD-II scanner and record P0871 and any related codes and freeze-frame data. Note conditions when code set.
Check transmission fluid level and condition; correct level and replace or flush if fluid is burnt/contaminated before further testing.
Perform a visual inspection of the sensor/switch C connector and wiring harness for damage, chafing, or corrosion; repair as needed.
Backprobe the sensor connector with the ignition on and monitor the signal while operating the transmission (or while commanded by scanner). Verify the signal type (analog voltage or digital switch) and that it changes with engine/transmission activity.
With ignition off, measure continuity and resistance between sensor pin(s) and TCM connector; check for short to ground or battery. Repair wiring faults found.
If wiring checks good, bench-test or replace the pressure sensor/switch C and retest. Use correct OEM or specified replacement.
If replacing sensor does not clear code, inspect valve body/pressure regulator and hydraulic passages for sticking or leakage; consider professional transmission inspection.
If hydraulic/mechanical components are confirmed good, verify TCM/PCM power, grounds, and update/reprogram or replace TCM only after all other causes have been eliminated.
After repairs, clear codes and road-test while monitoring live data to confirm correct pressure signal and no recurrence.

Likely causes

Damaged wiring or loose/corroded connector to pressure sensor C
Failed pressure sensor/switch C
Low/dirty transmission fluid causing incorrect pressure
Valve body or pressure regulator sticking or leaking
Intermittent TCM input/output fault

Fault status

⚠️ Status

Stored when the transmission control module detects the Fluid Pressure Sensor/Switch C circuit output is outside expected range, inconsistent with other inputs, or shows an open/short condition.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-4.0 hours