Hydraulic Oil Temperature Sensor Range/Performance
Diagnostic trouble code P0938 indicates that the powertrain control module (PCM) or transmission control module (TCM) has detected an abnormal or out-of-range signal from the hydraulic oil temperature sensor. The module sets this code when the sensor's voltage output does not correlate with expected values based on engine temperature and run time, suggesting a sensor malfunction or wiring issue.
While the vehicle can usually still be driven, an inaccurate transmission fluid temperature reading can lead to poor shift quality and potential long-term transmission wear. It should be diagnosed and repaired soon to prevent internal transmission damage.
Common symptoms
- Illuminated Check Engine Light
- Harsh or delayed transmission shifting
- Transmission may enter limp mode (restricted to a single gear)
- Torque converter clutch may not engage
- Decreased fuel economy
Likely causes
- Faulty hydraulic oil temperature sensor (transmission fluid temperature sensor)
- Corroded, damaged, or loose wiring and connectors in the sensor circuit
- Low or dirty transmission fluid
- Internal transmission mechanical failure causing abnormal fluid temperatures
- Faulty Powertrain Control Module (PCM) or Transmission Control Module (TCM) (rare)
What to check before replacing parts
Check the transmission fluid level and condition.
Look for: Fluid that is significantly low, burnt, or heavily contaminated can cause abnormal temperature readings and indicates a need for fluid replacement or further internal inspection.
Visually inspect the wiring harness and connectors for the hydraulic oil temperature sensor.
Look for: Look for frayed wires, melted insulation, corrosion in the connector pins, or loose connections that could cause high resistance or an open circuit.
Use a scan tool to monitor the live data for the hydraulic oil temperature sensor.
Look for: Compare the transmission fluid temperature reading to the engine coolant temperature when the vehicle is completely cold; they should be nearly identical. If the transmission temperature reads extremely high or low (e.g., -40 degrees or 300 degrees), the sensor or circuit is faulty.
Test the resistance of the hydraulic oil temperature sensor using a digital multimeter.
Look for: Compare the measured resistance at various temperatures to the manufacturer's specified resistance chart. If the resistance is out of specification or shows an open/short, the sensor is defective.
Check the reference voltage and ground at the sensor connector.
Look for: With the key on and engine off, the connector should typically show a 5-volt reference and a good ground. If either is missing, trace the circuit back to the PCM/TCM to locate the wiring fault.
This page explains the generic P0938 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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