In order to send temperature data back to the ECU, the hydraulic oil temperature sensor uses an electric signal that flows back to the engine computer through a series of wires and connectors. If there is an unusually low voltage signal detected on the hydraulic oil temperature sensor circuit, a P0939 DTC will be saved.
How Serious is the Code?
The P0939 diagnostic trouble code is very serious, because it means the ECU may not be able to prevent overheating and serious internal damage without reliable hydraulic oil temperature data.
Symptoms of a P0939 include:
- Check engine light
- Erratic vehicle behavior
- Limp Mode
- Corroded/broken connectors
- Damaged wiring
- Broken hydraulic oil temperature sensor
- Hydraulic fluid level is too low
- Hydraulic fluid is dirty
How to Diagnose the Code?
The P0939 OBDII trouble code diagnostic process should start with a check of the level and condition of the hydraulic oil itself (if it’s dirty, change it). The wiring and connectors along the circuit should also be carefully examined for signs of damage wires and corroded or broken connectors. Should no problems be found, then the hydraulic oil temperature sensor will need to be tested. The ECU itself may also need to be reprogrammed or replaced.
What Repairs Will Fix P0939?
- Repair damaged wiring and/or connectors
- Replace hydraulic oil temperature sensor
- Change hydraulic fluid
- Reprogram or replace ECU