Damaged wiring, poor connector contact or corrosion in the pressure sensor or switch circuit (open circuit, short to earth or short to supply); Faulty transmission fluid pressure sensor or switch; Low, degraded or contaminated transmission fluid, or fluid leaking into the connector or harness; Actual hydraulic pressure problem, such as a worn pump, blocked filter or valve body or solenoid fault; Fault inside the transmission control unit or its power and earth supply; Internal harness damage within the gearbox (on designs where the sensor is part of an integrated valve body or mechatronic unit).

P0840

Transmission Fluid Pressure Sensor/Switch A Circuit.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
8: AreaTransmission
40: FaultThe specific fault in that area

What it means

P0840 is the generic code for a fault in the transmission fluid pressure sensor or switch 'A' circuit. The transmission control unit monitors the signal from this sensor or switch, which reports hydraulic pressure in the automatic gearbox or CVT. It sets the code when the signal is out of range, stuck, erratic or does not match what the control unit expects for the commanded gear or state.

Symptoms

Warning light on, possibly with a transmission or gearbox warning message
Harsh, delayed or flared gear changes
Gearbox stuck in one gear (limp mode)
Slipping or poor acceleration
Torque converter lock-up not engaging, or higher fuel consumption
Transmission temperature or fluid warnings in some cases

Possible causes

Damaged wiring, poor connector contact or corrosion in the pressure sensor or switch circuit (open circuit, short to earth or short to supply)likely
Faulty transmission fluid pressure sensor or switchlikely
Low, degraded or contaminated transmission fluid, or fluid leaking into the connector or harnesslikely
Actual hydraulic pressure problem, such as a worn pump, blocked filter or valve body or solenoid faultpossible
Fault inside the transmission control unit or its power and earth supplyrare
Internal harness damage within the gearbox (on designs where the sensor is part of an integrated valve body or mechatronic unit)rare

How to diagnose

Read all stored and pending codes and the freeze frame data, noting gearbox temperature, gear selected and road speed when the fault set
Check the transmission fluid level and condition (where a dipstick or check procedure exists), looking for burnt smell, debris or metal particles; the procedure varies by model
Inspect the sensor or switch, its wiring and connector for chafing, corrosion, heat damage or fluid ingress
Check the control unit and sensor supply voltage, reference voltage and earth against the manufacturer's data
Watch the live pressure sensor or switch reading with the engine running and while selecting gears, and see whether it is plausible and changes as expected
Test the sensor signal circuit for continuity and shorts, and test the sensor itself against specification
Where available, carry out a line pressure test to confirm whether the actual hydraulic pressure is correct
Check for related codes such as solenoid, shift or ratio faults that point towards an internal problem

How to fix

Repair or replace damaged wiring and clean or replace faulty connectors
Replace the fluid pressure sensor or switch if it fails testing, using the correct fluid and procedure for the model
Correct the fluid level, repair any leaks, and change the fluid and filter if it is degraded or contaminated, as the manufacturer specifies
Repair or replace the pump, valve body or solenoids if pressure testing shows a real hydraulic fault
Repair the power or earth supply, or replace the transmission control unit, only once other causes are ruled out and the fault is confirmed
Clear the code after repair and road test to confirm it does not return

Can I keep driving?

Medium severity

The car can usually be driven carefully a short distance, but the gearbox may go into limp mode and low or incorrect pressure can cause internal damage. Have it checked soon and avoid heavy loads, towing or hard acceleration until it is diagnosed.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.