Error Code P0939 is defined as Hydraulic Oil Temperature Sensor Circuit Low. This is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II system, especially those made since 1996 up to the present. Of course, specifications on the definition, troubleshooting, and repairs vary from one make and model to another.
The hydraulic oil temperature sensor keeps an eye on the hydraulic fluid temperature. The manufacturer sets the heat level limit, and the sensor reports any fluctuations in heat levels in the Powertrain Control Module (PCM).
When Error Code P0939 appears, this means the Powertrain Control Module (PCM) detects an unusually low voltage signal coming from the hydraulic oil temperature sensor circuit.
This error code can show a range of different symptoms, such as:
- Erratic warning indicator lamps
- Overheating engine
- Erratic vehicle behavior
- The vehicle may go into “Limp mode”
The most common cause of this code is the sensor’s failure to report changes in the voltage back into the PCM, which could be a result of:
- Faulty wiring, corroded wires, damaged connectors
- Failed hydraulic oil temperature sensor
- Low or dirty fuel
- Failed PCM (rare)
How to Check
The first thing to diagnose this code is to connect the vehicle to an OBD-II code scanner to retrieve the error code and other possible error codes.
Since an electrical fault primarily causes this code, the diagnostic should start with the connectors and wiring of the hydraulic oil temperature sensor circuit. First, check the wirings and look for signs of damage, corrosion, wear and tear, or any signs of abnormality. Next, test the wires using voltmeter to gauge possible internal issues.
Also, the sensor itself must be checked as well.
How to Fix
Possible repairs include:
- Repair or replacement of damaged wirings and connectors
- Replacement of faulty hydraulic oil temperature sensor
- Hydraulic oil change
- Replacement and reprogramming of PCM (rare)
Since the hydraulic oil temperature sensor monitors how hot the hydraulic oil temperature is getting, this is a severe problem that must be addressed as soon as possible, as it could indicate overheating of the vehicle’s engine.
Always start the diagnosis for this code with the electrical components. PCM malfunction is rare, and replacing it can be expensive. Thus, it should be considered as the last resort option.