P0939
Hydraulic Oil Temperature Sensor Circuit Low
Transmission / hydraulic oil temperature
Generic OBD-II
Medium
What it means
P0939 means the transmission module is reading an abnormally low voltage (a stuck-low temperature) from the hydraulic oil temperature sensor, which monitors transmission fluid heat. The engine still starts and runs, but without a trustworthy temperature signal the module may alter shifting or enter limp mode, and it can't properly guard against overheating.
Medium
Can I keep driving? Usually driveable, but the module may adjust shift behaviour or enter limp mode without a valid temperature reading; fix it to avoid overheating going unnoticed.
Common symptoms
What you'll notice
- Check-engine light on
- Harsh or altered shift behaviour
- Transmission in limp / fail-safe mode
- Implausibly low oil-temperature reading on a scan tool
Common causes
Most likely first
- Short to ground in the temperature sensor signal wireLikely
- Faulty hydraulic oil temperature sensorCommon
- Wiring or connector fault at the sensorCommon
- Poor ground or reference for the sensor circuitLess often
- Failed transmission control module (least likely)Less often
How to diagnose & fix
Work through it in order
- Read freeze-frame data and view the oil-temperature value; a stuck-low reading confirms a circuit fault.
- Inspect the sensor connector and wiring for a short to ground, corrosion, or damage.
- Measure the sensor's resistance against spec at a known temperature.
- Repair wiring faults, then replace the temperature sensor if it's out of spec.
- Clear the code and confirm the reading rises as the transmission warms up.
FAQ
Quick answers
Is it safe to drive with P0939?+
Short trips are usually okay, but without a valid temperature signal the module can't protect the transmission from overheating, so repair it soon.
Can I just clear P0939?+
It returns until the sensor circuit is fixed, because the module keeps reading a signal that's stuck low.
Related codes
Often seen with P0939
Read & clear P0939 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX