P0871
Transmission Fluid Pressure Sensor/Switch 'C' Circuit Range/Performance
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The P0871 code means the Transmission Control Module has detected that the signal from transmission fluid pressure sensor/switch C is outside its expected operating range. This sensor monitors hydraulic pressure in a specific transmission circuit and reports that data back to the TCM, which uses it to make real-time adjustments to shift timing and clutch application pressure.
The important distinction with this code is determining whether the pressure is actually abnormal or whether the sensor is simply giving a bad reading. If the fluid level is correct and the transmission feels like it's shifting normally, the sensor itself is the most likely culprit. However, if you're also experiencing slipping, harsh shifts, or a burning smell, actual low pressure may be the root cause, which is a more serious concern.
Start by checking the transmission fluid level and condition. If the fluid is low, top it off and look for leaks. If the fluid is dark or burnt-smelling, a full fluid and filter service is needed. If the fluid is in good condition and at the proper level, the pressure sensor is likely faulty and needs replacement. The sensor is typically accessible from outside the transmission case (screwed into the valve body area), making it a moderately accessible repair. However, confirming the diagnosis with an actual pressure gauge reading is recommended before replacing the sensor.
Severity
Symptoms
- •Harsh or unpredictable shifting
- •Transmission slipping or flaring between shifts
- •Check Engine Light illuminated
- •Burning smell from overheated transmission fluid
- •Delayed engagement when shifting into gear
Likely Causes
The pressure sensor or switch is reading out of its expected range, sending inaccurate pressure data to the TCM. This may be due to an internal sensor failure or calibration drift.
Actual low pressure caused by low fluid level or degraded fluid properties triggers out-of-range readings from the pressure sensor.
Corroded, damaged, or loose wiring between the pressure sensor and TCM can cause signal distortion that falls outside the expected range.
A worn transmission pump may not generate consistent pressure, causing the sensor to read fluctuating or low values outside normal parameters.
Worn internal seals or gaskets can cause pressure drops in the specific circuit monitored by sensor C.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Replace transmission fluid pressure sensor/switch C
- Service transmission fluid and filter
- Repair wiring and connectors in the sensor circuit
- Replace the transmission pump if pressure is genuinely low
- Replace internal seals if a pressure leak is confirmed