Powertrain, generic · Transmission

P0840 Transmission Fluid Pressure Sensor/Switch A Circuit

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

Transmission fluid pressure sensor/switch A and B are inputs inside the automatic gearbox. A switch only reports pressure present or absent, a sensor reports a value. The letter is the maker's choice: Allison 1000 uses A and B for the switches on shift valves S1 and S2, GM's CVT for the primary and secondary pulley sensors, GM's 2025 8-speed A for the lube pressure sensor; Ford reports pressure faults under its solenoid codes instead. P0840 is the general circuit fault. GM documents it for the 2006 Allison: the module compares the commanded position of valve S1 with the switch feedback and retries the valve if they disagree; it then tells an electrical, mechanical or sporadic case apart. Type A, MIL after one trip. From 2007 Allison reports the same test under P0842/P0843.

Likely causes

Pressure switch or sensor A faulty or stuck
Sticking shift valve or dirt in the valve body
Harness or connector at the gearbox: fluid wicking, corrosion, chafing
Fluid level low or fluid contaminated
Transmission control module faulty

Symptoms

  • Check engine or transmission warning lamp on
  • Limp mode with a fixed gear, harsh or missing shifts
  • Torque converter clutch stays open, higher fuel use

Repair estimate

DifficultyHardWorkshop job
Parts and labour€150–900
Time1.5–5 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

P0841P0842P0843P0844P0845P0763

References: gsi.ext.gm.com, law.cornell.edu.