P0842Transmission Fluid Pressure Sensor/Switch A Circuit Low
- 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. 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.
- What you may notice
- Check engine or transmission warning lamp on
- Limp mode with a fixed gear, harsh or missing shifts
- CVT: ratio and power limited
- Likely causes
- LikelySignal wire shorted to ground or, on GM duty-cycle sensors, open
- LikelySwitch stuck closed or sensor faulty internally
- Possible5 V supply of the sensor missing
- PossibleShift valve stuck in the applied position, so the switch reports pressure correctly
- RareInput of the control 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.