P0843: Transmission Fluid Pressure Sensor/Switch A Circuit High

What does P0843 mean?

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.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

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

Common causes

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

Likelihood is a general estimate across vehicles; your make and model may differ.