Hydraulic Oil Temperature Sensor Circuit High
Diagnostic trouble code P0940 indicates that the powertrain control module (PCM) or transmission control module (TCM) has detected a high voltage signal from the hydraulic oil temperature sensor circuit. This typically means the sensor is reporting an unusually low temperature or there is an open circuit in the sensor's wiring.
While the vehicle may still be drivable, a faulty hydraulic oil temperature reading can prevent the transmission from shifting properly or entering overdrive, potentially leading to poor performance or overheating if ignored.
Common symptoms
- Illuminated Check Engine Light
- Harsh or delayed transmission shifting
- Transmission may not shift into the highest gear (overdrive)
- Decreased fuel efficiency
- Transmission temperature warning light illuminated (on some vehicles)
Likely causes
- Faulty hydraulic oil temperature sensor
- Open or shorted wiring in the hydraulic oil temperature sensor circuit
- Corroded, damaged, or loose electrical connector at the sensor
- Low or dirty transmission fluid affecting sensor operation
- Faulty Transmission Control Module (TCM) or Powertrain Control Module (PCM) (rare)
What to check before replacing parts
Check the transmission fluid level and condition.
Look for: Ensure the fluid is at the correct level and is not burnt, heavily contaminated, or excessively dirty, which can affect sensor readings.
Visually inspect the hydraulic oil temperature sensor wiring and connector.
Look for: Look for broken wires, melted insulation, or a corroded/loose connector that could cause an open circuit or high resistance.
Use a scan tool to monitor the hydraulic oil temperature sensor data PID.
Look for: Check if the temperature reading is stuck at an extreme low value (e.g., -40 degrees) or does not change as the transmission warms up, indicating an open circuit or failed sensor.
Test the sensor's resistance using a digital multimeter.
Look for: Compare the measured resistance at various temperatures to the manufacturer's specifications. An infinite resistance reading indicates an internally open sensor.
Check the reference voltage and ground at the sensor connector.
Look for: With the key on and engine off, verify that the PCM/TCM is supplying the correct reference voltage (usually 5V) and a good ground to the sensor.
This page explains the generic P0940 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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