Powertrain, generic · Transmission

P0878 Transmission Fluid Pressure Sensor/Switch D Circuit High

Check engine lightUsually off
EmissionsNot relevant
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. 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.

Likely causes

Signal wire or connector open (gearbox pass-through, harness)
Pressure switch does not close (diaphragm, seal)
Clutch pressure does not build: low fluid, leak, regulator valve
Gearbox module input

Symptoms

  • 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)

Repair estimate

DifficultyHardWorkshop job
Parts and labour€200–1,500
Time1–5 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

P0875P0876P0877P0873P0990

References: gsi.ext.gm.com, gsi.ext.gm.com, gsi.ext.gm.com.