Powertrain, generic · Transmission

P0843 Transmission Fluid Pressure Sensor/Switch A Circuit High

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. P0843 means the signal is too high. For a switch that is 'open although pressure should be there': Allison sets it when switch 1 stays without pressure for over 0.07 s with valve S1 applied, type A; a 2006 GM four-speed used switch A for the converter clutch release and set it after 6 s, type B. For a sensor it is a short to voltage or open: GM's CVT above 95 %, its 8-speed above 91 % duty cycle.

Likely causes

Signal or ground wire open, connector soaked with fluid
Switch stuck open or sensor faulty
Signal wire shorted to 5 V or battery voltage
Too little pressure at the switch: low fluid level or sticking valve
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€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

P0840P0841P0842P0848P0741

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