Fault codeP0840

MODERATE

Transmission Fluid Pressure Sensor/Switch A Circuit

What this code means

Transmission Fluid Pressure Sensor/Switch A Circuit.

P0840 is a generic code: it means the same thing on every make. P0840 belongs to family P08xx — Transmission (continued). Its page explains how to read a code and groups all its codes by part.

Type of fault

The control unit has detected an electrical anomaly on this circuit without saying exactly which. Diagnosis starts with a visual check of the wiring and connector, then measuring the component — resistance, supply, ground — with a multimeter.

Gearbox hydraulic pressure

Automatic, dual-clutch and automated gearboxes work with hydraulic pressure, produced by a pump and measured by pressure sensors or switches. The control unit checks that the pressure follows what it commands.

What you notice

Harsh or slipping shifts, a gearbox that will not engage, a limp mode with a single gear.

How to check it

Check the fluid level and condition first. Then the connector and wiring of the sensor or switch. A pressure that is really too low points to the pump, a filter or an internal leak, a job for a gearbox specialist.

What to do

Have it checked this week. The car remains usable, but the fault will not clear itself and will fail the roadworthiness test.

Clearing a code switches the light off but repairs nothing: if the fault remains, the code comes back. Clearing a fault code yourself →

How to read this code

PPowertrain
0standardised code
8Transmission (continued)
40P0840