What it means
The hydraulic oil temperature sensor reports a circuit low fault. Oil temperature feedback for hydraulic system is unreliable.
At a glance
- Usually lights the check-engine light
How it works
The hydraulic oil temperature sensor feeds fluid temperature information to the TCM. P0939 sets when the sensor circuit signal is lower than expected, which may result from a sensor fault, a circuit problem, or a poor connector contact.
Common causes
Most likely first.
- Temperature sensor failed
- Wiring fault
- Connector pins corroded - high contact resistance
Symptoms
- Check engine or transmission warning light on
- Harsh, delayed, or missing shifts
- Possible limp mode (locked in one gear)
- Slipping or shuddering during gear changes
How to diagnose it
- Save freeze-frame data and compare the live oil temperature reading with ambient temperature when the transmission is cold to see whether the signal is implausibly low.
- Inspect the temperature sensor connector for corrosion or loose pins and check the harness for damage or a short to ground.
- Check the signal, reference or supply, and ground circuits against the manufacturer's wiring information, paying particular attention to voltage pulled low by a short or poor contact.
- If the wiring and connector are sound, test the temperature sensor according to the manufacturer's procedure.
- Repair the confirmed fault, then confirm the live reading responds plausibly as the transmission warms and the code stays cleared.
Common fixes
- Clean and secure corroded or loose temperature sensor connector pins.
- Repair a signal wire shorted to ground or other damaged circuit wiring.
- Replace the temperature sensor if it produces a low signal with a sound circuit.
Can I keep driving?
If shifting is normal, limit driving and avoid heavy loads because the TCM cannot rely on the oil temperature reading. Stop if the transmission slips, shudders, or shifts harshly, as continued operation may worsen transmission damage.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.