P0843Transmission Fluid Pressure Sensor/Switch A Circuit High
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- 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.
- What you may notice
- 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
- Likely causes
- LikelySignal or ground wire open, connector soaked with fluid
- LikelySwitch stuck open or sensor faulty
- PossibleSignal wire shorted to 5 V or battery voltage
- PossibleToo little pressure at the switch: low fluid level or sticking valve
- RareControl module faulty
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Hard · Usually a job for a workshop.Typical cost: €120 – €900
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.