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

What does P0871 mean?

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.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

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

Common causes

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

Likelihood is a general estimate across vehicles; your make and model may differ.