P0939 Oil temperature sensor: signal or voltage too low (hydraulic oil)

By: Diagno editorial team · Updated: 2026-10-15

P0939 means: Oil temperature sensor: signal or voltage too low (hydraulic oil). A temperature sensor tells the computer the temperature of the air, coolant, fuel or oil. Drive gently and have the car checked soon. The repair usually costs about €30–200 in Estonia. The most common cause: connector loose, damp or corroded.

What it means

A temperature sensor tells the computer the temperature of the air, coolant, fuel or oil. With a wrong reading the computer gets the mixture and the fan wrong. The signal voltage is too low. Usually a wire is broken, a connector is loose or a wire is shorted to ground.

What you may notice

• Engine light on
• Hard cold starts
• The fan may run all the time
• Higher fuel consumption

What could be wrong

1. 1Connector loose, damp or corroded
2. 2Wire broken or shorted to ground
3. 3Faulty sensor
4. 4Engine computer fault (rare)

Repair difficulty: Easy

What you can check yourself

First look at the connector and wiring: anything loose, melted or green with corrosion? Check the sensor connector and make sure it is clipped in. On a cold engine, compare the reading with the outside temperature. You can do the simpler checks yourself.

How we fix it

1. We read the fault data on the diagnostic tool and check whether the fault is constant
2. We measure the circuit's supply, ground and signal
3. We repair the wire or replace the connector
4. If the wiring is fine, we replace the sensor

Standard SAE description: Hydraulic Oil Temperature Sensor Circuit Low