Powertrain, generic · Transmission

P0870 Transmission Fluid Pressure Sensor/Switch C Circuit

Check engine lightOn
EmissionsRelevant
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. P0870 is the general fault. Allison's 2006 table uses it for the switch on valve S3: if it reports pressure for more than 20 ms with the valve at rest, the valve is retried twice and the case classed as electrical, mechanical or sporadic, type A. Ram corrected false P0870 'hydraulic pressure tests' on the 2019 68RFE with a software update.

Likely causes

Pressure switch C faulty (sticking, seal leaking)
Sticking shift valve or worn valve body bore
Wiring or connector at the gearbox
Control module software (Ram 68RFE)
Low fluid level

Symptoms

  • MIL or transmission warning lamp on
  • Limp mode with a fixed gear
  • Harsh or missing shifts

Repair estimate

DifficultyHardWorkshop job
Parts and labour€100–900
Time0.5–5 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

P0871P0872P0873P0874P0875P0840

References: gsi.ext.gm.com, static.nhtsa.gov, law.cornell.edu.