What it means
The transmission fluid pressure sensor/switch C reports a circuit range/performance fault. Hydraulic pressure feedback for shift control is unreliable on this channel.
At a glance
- Usually lights the check-engine light
- The engine or gearbox may go into limp mode
How it works
Transmission pressure sensor/switch C provides the control module with feedback about hydraulic pressure in the transmission. The module compares that signal with expected pressure behavior, and sets P0871 when the C-channel reading is electrically plausible but outside the expected range or does not track as expected.
Common causes
Most likely first.
- Sensor offset drifted
- Aerated fluid affects pressure
- Sensor element aged beyond calibration window
Symptoms
- Check engine or transmission warning light on
- Harsh, delayed, or missing shifts
- Possible limp mode (locked in one gear)
- Slipping or shuddering during gear changes
How to diagnose it
- Record freeze-frame data and compare live pressure-sensor C data with commanded gear and other pressure channels to see when its reading becomes implausible.
- Check the transmission fluid level and condition using the vehicle's specified procedure, since aerated or unsuitable fluid can disturb pressure feedback.
- Inspect sensor C, its connector, and nearby harness for damage, contamination, loose pins, or corrosion that could skew the signal.
- With a scan tool and wiring information, check whether sensor C's reading changes smoothly as operating conditions change.
- Test the C sensor signal and its reference and ground circuits against the manufacturer's specification, then verify whether the sensor or hydraulic pressure is responsible for the mismatch.
Common fixes
- Correct the fluid level or condition if it is aerated or otherwise unsuitable.
- Clean and secure the sensor C connector and repair damaged wiring or terminals.
- Replace pressure sensor/switch C if its signal is offset or fails to track correctly.
- Address a confirmed hydraulic pressure problem if sensor C tests correctly.
Can I keep driving?
Avoid towing, hard acceleration, and long trips if shifts are harsh, delayed, slipping, or the transmission enters limp mode. Continued driving with unreliable C-channel feedback can worsen clutch or transmission damage; stop if drive is lost or slipping becomes severe.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.