P0878Transmission Fluid Pressure Sensor/Switch D Circuit High
- Generic OBD-II code · Powertrain
- Can trigger limp mode
- How the car detects it
- 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. P0878 means switch D reports no pressure although pressure is commanded. GM checks the 1-2-3-4 clutch above 700 kPa and sets it after 6 counts on the 6L80 or 8 on the 6T70, without MIL. Allison 2007 sets it when the reverse switch stays open in R or fails its check after engine stop, type A.
- What you may notice
- Code stored in the gearbox module; on GM without MIL, on Allison with MIL
- Slip or shift shock in gears 1 to 4 (GM) or in reverse (Allison)
- Likely causes
- LikelySignal wire or connector open (gearbox pass-through, harness)
- PossiblePressure switch does not close (diaphragm, seal)
- PossibleClutch pressure does not build: low fluid, leak, regulator valve
- RareGearbox module input
- Repair
- Hard · Usually a job for a workshop.Typical cost: €200 – €1,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.