P0872: Transmission Fluid Pressure Sensor/Switch C Circuit Low

What does P0872 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. P0872 means switch C reports pressure while its clutch is exhausted. GM checks it only when the 2-6 brake is commanded below 50 kPa, after a temperature-dependent delay, and counts one fault per clutch change: 10 on the 6L80, 18 on the 2010 6T70; listed as 'special no trip', without MIL. Allison 2007 sets it when switch 3 reports pressure for over 20 ms with the valve at rest, type A.

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

Symptoms

• Code stored in the gearbox module; on GM without MIL, on Allison with MIL
• Harsh shifts, possibly substitute gear

Common causes

• High Pressure switch stuck closed or shorted internally
• Medium Signal wire shorted to ground inside the gearbox
• Medium Clutch circuit does not exhaust: dirt, sticking valve
• Low Gearbox module

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