Fault codeP0939

MODERATE

Hydraulic Oil Temperature Sensor Circuit Low Input

What this code means

Hydraulic Oil Temperature Sensor Circuit Low Input.

P0939 is a generic code: it means the same thing on every make. P0939 belongs to family P09xx — Transmission and controls. Its page explains how to read a code and groups all its codes by part.

Type of fault

The control unit reads a voltage on this circuit below its expected range. Usual causes: a wire shorted to ground or broken, a corroded or loose connector, or the component itself. Check the wiring and connector before replacing the part — that is where many of these faults turn out to be.

Gearbox fluid temperature sensor

This sensor, usually inside the gearbox, measures the temperature of the transmission fluid. The control unit uses it to adapt shift quality, the lock-up of the torque converter and to protect the gearbox when the fluid overheats.

What you notice

Harsh or late shifts, especially when cold, the torque converter not locking up, sometimes a gearbox in limp mode.

How to check it

Check the fluid level and condition first: dark, burnt-smelling fluid is a sign of overheating. Then the connector on the gearbox, where leaks of fluid can wick into the wiring. On many gearboxes the sensor is part of an internal wiring harness and replacement requires removing the sump.

Watch it live in Klaro

With an OBD2 adapter plugged in, these live readings show the part at work, when your car reports them:

Coolant temperature
Vehicle speed

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
9Transmission and controls
39P0939