P0878 code meaning
Transmission Fluid Pressure Sensor/Switch D Circuit High
What P0878 means
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.
P0878 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modePossible
- Repair difficultyHard
- Typical labor1 to 5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P0878
- 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)
Common causes of P0878
- Most 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
- Less likelyGearbox module input
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Transmission fluid pressure sensorTransmission valve bodyTransmission wiringTransmission control module
What each part of P0878 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 8 | Area: transmission. |
| 78 | The exact fault in that area. |
What to do next
- Note P0878 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0878
What does P0878 mean?
P0878 means "Transmission Fluid Pressure Sensor/Switch D Circuit High". Transmission fluid pressure switches C and D report whether a clutch circuit is pressurised. It is a generic code, so it means the same on every make.
Is it safe to drive with P0878?
Drive gently and only to a workshop. With P0878 the car can go into limp mode (reduced power) to protect itself.
Will P0878 fail an emissions test?
P0878 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is P0878 to fix?
Difficulty: hard. A typical repair takes 1 to 5 hours of work. This is a job for a workshop.
Will P0878 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.