OBD-II trouble code
P0938: Hydraulic Oil Temperature Sensor Range/Performance
The hydraulic oil temperature signal is electrically healthy but the value it reports is not believable. Unusually for a temperature code, there is a second thermometer in the same gearbox to check it against.
Quick facts
- System
- Powertrain
- Category
- Transmission
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $80 – $1,800
- DIY difficulty
- Shop recommended
Browse every code in P0900–P0999, or start from the full code library.
What does P0938 mean?
P0938 is not a broken-wire code. The circuit is intact, the voltage sits inside its legal window, and the module can read a number — it simply does not believe the number. Range/performance faults are always the module comparing one input against something else it knows, and what makes this particular code unusually tractable is what that something else is.
Most temperature range/performance codes leave a technician arguing with a single reading. This one does not, because the transmissions that carry a hydraulic oil temperature sensor almost always carry a second temperature sensor as well, watching the gear oil. Those two fluids are in the same aluminium case, bolted to the same engine, sitting in the same airflow. They are not the same temperature in service — the actuator circuit runs its own thermal cycle — but after a vehicle has stood overnight they must both be at ambient, and they must both climb once the car is driven. Two thermometers in one gearbox is a second opinion that no engine-side temperature family offers, and it converts an argument about specification tables into a straight comparison anybody can read on a scan tool.
The other half of the diagnosis is about speed rather than value, and it comes from a physical fact about the circuit. The hydraulic actuator system holds a small volume of fluid — litres in a gearbox sump versus something closer to a large drink can in the actuator circuit — and a small volume changes temperature fast. It should warm quickly after a cold start, and it should respond visibly to a few minutes of stop-start clutch work. A reading that is plausible in absolute terms but moves like a gearbox sump is the classic signature here: the thermistor has gone lazy, or it has lost thermal contact with the fluid because the level has dropped away from it. Watching the trace move is often more diagnostic than reading any single value off it.
The practical consequence of the fault is modest, because the module already knows the input is unreliable and will fall back to a substitute. Shift quality goes slightly off, adaptation stops settling, and the car keeps driving. What P0938 actually costs the owner is the protection and compensation the real reading would have provided — which is why it is worth fixing even though nothing dramatic happens if it is not.
One caution before condemning the sensor. A range/performance verdict depends on the module's expectations being right, and those expectations are wrong if the vehicle has just had an unusual thermal history — a long recovery on a trailer in winter, a night in a heated workshop, or a battery disconnection that wiped the learned values. Clear the code, drive the car through a genuine cold-to-warm cycle, and see whether it comes back before spending anything.
Common causes
- Thermistor drifted out of calibration but still inside its electrical limits
- Sensor responding sluggishly through contamination or a coating on the tip
- Low hydraulic fluid level leaving the sensor poorly wetted
- High resistance in a connector adding a fixed offset to the reading
- Wrong or non-specification hydraulic fluid altering the thermal response
- Corroded ground return shifting the measured voltage
- Aftermarket or incorrect-part sensor with a different resistance curve
- Damaged sensor tip after previous actuator or clutch work
- Software or adaptation values not reset after a component was replaced
- Control module input drift on a very high mileage unit
Symptoms
- Check engine light with a stored transmission fault and no obvious driving complaint
- Reported hydraulic oil temperature disagreeing with the gear oil temperature after an overnight stand
- Temperature reading that climbs far more slowly than the driving would suggest
- Slightly inconsistent shift quality that never settles into a pattern
- Transmission adaptation values that will not complete
- No overheat warning during genuinely hard use
- Reading that appears frozen at one value for long periods
- Fault returning only after a full cold-to-warm cycle
- Occasional delayed engagement when pulling away
- Fault appearing shortly after a transmission or clutch repair
Diagnostic steps
- 1.After the vehicle has stood overnight, read the hydraulic oil temperature and the gear oil temperature together — both should be at ambient and both should agree closely.
- 2.Drive the car and watch both temperatures climb, because a hydraulic circuit that lags the gearbox sump is behaving backwards for its fluid volume.
- 3.Log the reading through several minutes of stop-start clutch work, where a healthy sensor in this small a fluid volume should respond visibly.
- 4.Check the hydraulic fluid level, since a sensor that is only partly wetted reads the air above the fluid rather than the fluid.
- 5.Measure sensor resistance at a known temperature and compare it against the manufacturer's curve rather than against a generic thermistor table.
- 6.Confirm the fitted sensor is the correct part number, because a substitute with a different curve reads plausibly and wrongly.
- 7.Voltage-drop the signal and ground sides to rule out a fixed offset introduced by connector resistance.
- 8.Ask whether the vehicle has had an unusual thermal history recently, then clear and re-drive before spending money.
- 9.Confirm any adaptation reset the manufacturer requires after a previous component replacement was actually performed.
- 10.Compare the reading against a hand-held infrared measurement of the actuator housing as an independent physical check.
Repair cost
$80 – $1,800
Diagnosis is $80 to $220, and on this code a decent share of that is simply logging two temperatures through a cold-to-warm cycle. A wiring or connector repair is $110 to $360. A separately supplied hydraulic oil temperature sensor runs $110 to $520 fitted. Where the sensor is built into the actuator or mechatronic assembly the job becomes $900 to $1,800. A hydraulic fluid top-up or service, if the level is the cause, is $150 to $400 and is the cheapest outcome available here.
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DIY vs shop
Leave this one to a qualified shop. It typically involves emissions-critical components, refrigerant handling, or other work that requires manufacturer-grade tooling, training, or certification. DIY attempts often produce a more expensive problem than the original code.