P0938Generic code

P0938 Code: Hydraulic Oil Temperature Sensor Circuit Range/Performance

The P0938 code means the hydraulic oil temperature sensor gives a reading that is electrically valid but does not match what the computer expects. Shifts can turn slower or harsher. A multimeter rarely finds this fault, so you compare the reading with the real condition.

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 P0938 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 P0938 code is a range fault: the sensor is measuring, but measuring wrong, or the system really is running outside its normal range.

Without a trustworthy signal the computer works from a fixed value, and shifts can be slower or harsher when the car is cold.

This kind of fault usually does not show up with a multimeter, because everything looks fine electrically. You have to compare what the part reports with reality: use a gauge, a thermometer or a tachometer, or command the component with the scan tool and see whether the physical result matches the order.

P0938 code symptoms

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

Likely causes of the P0938 code

  1. Sensor that has aged or drifted. It gives believable readings that are wrong.
  2. A real abnormal condition. The system is truly running outside its normal range.
  3. High resistance in the connector. Corrosion shifts the reading.
  4. Port, hose or tone wheel that is damaged or blocked, depending on the type of sensor.

How to diagnose the P0938 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 watch whether it follows the real temperature.
  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 P0938 code

  • Repair the connector or the wiring.
  • Replace the sensor if the tests confirm it has drifted.
  • Correct the real condition if there is one.
  • Clear the P0938 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

Range faults are not tied to one make, but this type of automated gearbox sets them often. Many hydraulic systems call for a specific actuator fluid, and the reservoir should be checked regularly.

P0938 on Opel

Very common on the Opel Easytronic.

P0938 on Fiat

Very common on the Fiat Dualogic.

P0938 on Toyota

Very common on the Toyota MMT.

P0938 on Smart

Very common on Smart cars with this type of gearbox.

P0938 on BMW

Very common on the BMW SMG.

Frequently asked questions about P0938

Can I drive with a P0938 code?
Yes, with caution. The gearbox keeps working but shifts may be slower or harsher, so have it checked.
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 P0938?
As a rough guide, between $40 and $400. The final bill depends on whether it is a connector, a wire or the sensor itself.
What does the P0938 code mean?
It means the oil temperature reading is plausible but does not agree with what the computer expects. The sensor may be measuring badly, or the system may really be outside its normal range.
Will a P0938 code clear itself?
Only if the reading returns to normal and stays there for several drive cycles. If a sensor has drifted, the code will come back.

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