Powertrain, generic · Transmission

P0842 Transmission Fluid Pressure Sensor/Switch A Circuit Low

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. P0842 means the signal is too low. For a switch that is 'closed although no pressure is expected': Allison sets it when switch 1 reports pressure for over 0.1 s with valve S1 at rest, type A. For a sensor it is a short to ground: GM's CVT below 3 % duty cycle, its 8-speed below 9 % (type B, two trips), Mitsubishi below 0.09 V for 5 s.

Likely causes

Signal wire shorted to ground or, on GM duty-cycle sensors, open
Switch stuck closed or sensor faulty internally
5 V supply of the sensor missing
Shift valve stuck in the applied position, so the switch reports pressure correctly
Input of the control module faulty

Symptoms

  • Check engine or transmission warning lamp on
  • Limp mode with a fixed gear, harsh or missing shifts
  • CVT: ratio and power limited

Repair estimate

DifficultyHardWorkshop job
Parts and labour€120–900
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

P0840P0841P0843P0847P0741

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