P0939Generic code

P0939 Code: Hydraulic Oil Temperature Sensor Circuit Low

The P0939 code means the signal from the hydraulic oil temperature sensor is below its minimum value. It is usually a signal wire shorted to ground or a missing reference voltage. The effect is shifting that is slower or harsher, mostly when the car is cold.

Severity
Medium
Can I keep driving?
Yes, but with caution
Estimated cost
$40 to $400
Difficulty
Medium
System
Transmission
Applies to
All OBD2 vehicles

Updated Reviewed by Renzo Chacca

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What the P0939 code means

Automated manual transmissions (Opel Easytronic, Fiat Dualogic, Toyota MMT, BMW SMG, Ford Durashift, Volkswagen ASG) and many dual-clutch gearboxes use electric or hydraulic actuators to release the clutch, pick the gear plane (selection) and engage the gear (engagement). The transmission computer watches every actuator through position sensors. The temperature of the hydraulic fluid changes how thick it is, and the computer uses that temperature to adjust the timing of each actuation.

The sensor changes its resistance with temperature and the computer reads the resulting voltage. The P0939 code is a low signal. It usually comes from the signal wire shorting to ground, or from the reference voltage going missing.

Without this signal the computer uses a fixed value, and shifts can be slower or harsher on a cold car.

To confirm it, unplug the component with the key on and watch what the reading or the code does. If the value jumps to the opposite extreme, the wiring is fine and the component is the one pulling the signal to ground. If nothing changes, look for the short in the wire, starting where the harness rubs against the engine, the chassis or the exhaust.

P0939 code symptoms

  • Check engine light on.
  • Shifts that are slower or harsher than normal.

Likely causes of the P0939 code

  1. Signal wire shorted to ground. The harness rubs against the engine, the chassis or a bracket.
  2. Shorted reference supply. If several sensors fail at once, suspect the shared reference.
  3. Damaged sensor with an internal short.
  4. Connector with water or corrosion. It creates a path to ground.

How to diagnose the P0939 code step by step

  1. Read the sensor value on the scan tool and compare it with the real fluid temperature.
  2. Inspect the connector for corrosion, water or loose pins.
  3. Check the supply and ground with the key on: the reference should be correct and the ground should drop less than 0.1 volt.
  4. Measure the signal with the connector plugged in and see whether it changes with the condition.
  5. Do the wiggle test on the harness with the scan tool in graph mode.
  6. Check continuity of each wire to the computer, which should be under 1 ohm.

How to fix the P0939 code

  • Repair the connector or the wiring.
  • Replace the sensor if the tests confirm the short is inside it.
  • Correct the real condition if there is one.
  • Clear the P0939 code and verify with a road test.

Technical reference data

  • NTC thermistor: its resistance drops as the fluid gets hotter.
  • Reference values only: check the manufacturer data for your vehicle.

Brand-specific notes

The make list below is where this family of codes is seen most. In many hydraulic systems the actuator fluid is a specific one, so keep an eye on the reservoir.

P0939 on Opel

Very common on the Opel Easytronic.

P0939 on Fiat

Very common on the Fiat Dualogic.

P0939 on Toyota

Very common on the Toyota MMT.

P0939 on Smart

Very common on Smart cars with this style of transmission.

P0939 on BMW

Very common on the BMW SMG.

Frequently asked questions about P0939

Can I drive with a P0939 code?
Yes, with caution. Shifts may be slower or harsher, but the car normally keeps running.
Does the sensor have to be replaced?
Only if the tests confirm it. The connector and the wiring fail just as often as the sensor does.
What does the hydraulic oil temperature sensor do?
Automated manual transmissions (Opel Easytronic, Fiat Dualogic, Toyota MMT, BMW SMG, Ford Durashift, Volkswagen ASG) and many dual-clutch gearboxes use electric or hydraulic actuators to release the clutch, pick the gear plane (selection) and engage the gear (engagement). The transmission computer watches each actuator with position sensors. The temperature of the hydraulic fluid changes its thickness, and the computer uses it to adjust the timing of each actuation.
How much does it cost to fix P0939?
As a rough guide, between $40 and $400. Fixing a rubbed wire is at the low end, and a new sensor with labor is higher.
What does the P0939 code mean?
It means the oil temperature signal is lower than the minimum the computer allows. That points to a short to ground or a missing reference voltage.
What is the most common cause of P0939?
A signal wire shorted to ground where the harness rubs on the engine, chassis or a bracket. Water in the connector can do the same thing.

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