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.
Medium severity. Can I keep driving?Drive gently to a repair shop. If it slips, you hear clunks, or it loses its gears, do not keep driving.
Symptoms
- Check engine or transmission warning lamp on
- Limp mode with a fixed gear, harsh or missing shifts
- CVT: ratio and power limited
Most 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
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: hard
- It can trigger limp mode.
- It affects emissions.
What each character of P0842 means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 8: Transmission
- 42: Specific fault in that system
Frequently asked questions
Can I keep driving with code P0842?
Drive gently to a repair shop. If it slips, you hear clunks, or it loses its gears, do not keep driving.
What causes code P0842?
The most common causes are: 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.
Related codes
- P0840Transmission Fluid Pressure Sensor/Switch A Circuit
- P0841Transmission Fluid Pressure Sensor/Switch A Circuit Range/Performance
- P0843Transmission Fluid Pressure Sensor/Switch A Circuit High
- P0847Transmission Fluid Pressure Sensor/Switch B Circuit Low
- P0741Torque Converter Clutch Circuit Performance/Stuck Off
Sources
- gsi.ext.gm.com: 23OBDG03A Part1
- gsi.ext.gm.com: 25OBDG06A
- gsi.ext.gm.com: 07 GRP15 Allison 1000
- gsi.ext.gm.com: 2006trans5 b
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.