Powertrain, generic · Transmission

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

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

Transmission fluid pressure switches C and D report whether a clutch circuit is pressurised. They are more widespread than A and B: GM's 6L80/6L90 and 6T40/6T70 six-speeds put switch C on the 2-6 brake and D on the 1-2-3-4 clutch, Allison uses them for its third shift valve and its reverse switch, Ram's RFE gearboxes for the clutch pressure switches in the solenoid block. Ford reports pressure faults under its solenoid codes. P0871 means the switch does not follow the command or gear. On Ram's RFE it is the overdrive clutch switch: open in gears 1 to 3, closed in 4 to 6; it closes at about 1.5 bar and opens at 0.75 bar. Sonnax blames worn valve body bores in about 90 % of cases; on 2024 trucks it came with P0868 from a broken rear thrust washer. Allison 2006: switch without pressure 5 s after command, type A.

Likely causes

Worn valve body bores or shift valve plugs
Mechanical damage: thrust washer, clutch seals
Pressure switch faulty (lip seal)
Dirt or sticking valve (dual clutch)

Symptoms

  • MIL or transmission warning lamp on
  • Limp mode, often in 2nd or 3rd gear
  • Harsh engagement, neutral feeling in 4th gear

Repair estimate

DifficultyHardWorkshop job
Parts and labour€200–3,500
Time1–12 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

transmission fluid pressure sensortransmission valve bodytransmission wiringtransmission control module

Related codes

P0870P0872P0873P0876P0868P0841

References: sonnax.com, gearsmagazine.com, static.nhtsa.gov, gsi.ext.gm.com.