P0840Transmission Fluid Pressure Sensor/Switch A Circuit
- 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. P0840 is the general circuit fault. GM documents it for the 2006 Allison: the module compares the commanded position of valve S1 with the switch feedback and retries the valve if they disagree; it then tells an electrical, mechanical or sporadic case apart. Type A, MIL after one trip. From 2007 Allison reports the same test under P0842/P0843.
- 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
- LikelyPressure switch or sensor A faulty or stuck
- PossibleSticking shift valve or dirt in the valve body
- PossibleHarness or connector at the gearbox: fluid wicking, corrosion, chafing
- PossibleFluid level low or fluid contaminated
- RareTransmission 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: €150 – €900
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.