P0940
UniversalHydraulic Oil Temperature Sensor Circuit High Input
Medium SeverityThe P0940 code indicates that the Powertrain Control Module (PCM) has detected an abnormally high voltage reading from the hydraulic oil temperature sensor circuit. This usually means the PCM is receiving a signal suggesting the hydraulic oil is extremely hot, even if it's not, which can lead to improper shifting or damage to the transmission or other hydraulic systems if left unaddressed.
⚠️ Before You Begin (Safety First)
Always allow the engine and hydraulic systems to cool completely before attempting any work. Disconnect the battery before inspecting or repairing electrical components to prevent accidental shorts or injury. Use proper jack stands when working under the vehicle. Be aware that hydraulic fluid can be hot and under pressure.
Common Symptoms
- Check Engine Light illuminated
- Erratic or harsh transmission shifting
- Transmission may go into 'limp mode'
- Delayed engagement of gears
- Overheating warning for transmission (if equipped)
Common Causes
- Faulty hydraulic oil temperature sensor
- Corroded, damaged, or open wiring/connectors in the sensor circuit
- Short to voltage in the sensor circuit
- Faulty Powertrain Control Module (PCM), though rare
Frequently reported on: Ford F-Series Super Duty (2005-2010), Chevrolet Silverado/GMC Sierra HD (2001-2007), Dodge Ram Cummins (2003-2007), Jeep Grand Cherokee (2005-2010), Various heavy-duty applications with automatic transmissions
Diagnostic Steps
- Inspect the wiring and connectors leading to the hydraulic oil temperature sensor for any signs of damage, corrosion, or loose connections. Repair as needed.
- Locate the hydraulic oil temperature sensor (often on the transmission or a hydraulic manifold). Disconnect its electrical connector.
- Using a multimeter, test the resistance of the sensor at different temperatures (if accessible and safe to heat gently, or by comparing to a known good sensor). An extremely high or infinite resistance may indicate a faulty sensor.
- Check for voltage at the sensor connector from the PCM. There should typically be a reference voltage (e.g., 5V) and a good ground. A lack of reference voltage or ground could indicate a wiring issue back to the PCM.
- If the sensor and wiring appear good, consider replacing the hydraulic oil temperature sensor.
- Clear the DTCs and test drive the vehicle to see if the code returns.
👨🔧 When to Call a Professional
If diagnostic steps confirm the hydraulic oil temperature sensor is faulty, but the issue persists after replacement, or if you suspect internal transmission problems, it's best to consult a professional transmission specialist. Complex electrical diagnostics or transmission disassembly are beyond typical DIY scope.
Commonly Asked Questions
What does 'high input' mean for a sensor circuit?
A 'high input' usually means the PCM is receiving a voltage signal from the sensor that is higher than expected, indicating a very high temperature or an open circuit/short to voltage, even if the actual temperature isn't that high.
Can I drive with a P0940 code?
It is not recommended to drive for extended periods with this code. The transmission relies on accurate temperature readings for proper shifting and to prevent overheating. Ignoring it can lead to more severe and costly transmission damage.
Where is the hydraulic oil temperature sensor located?
The hydraulic oil temperature sensor is typically located on the transmission casing itself, often near the valve body or within the transmission pan, where it can directly monitor the fluid temperature.
Is it possible for a low fluid level to cause this code?
While low fluid can cause transmission overheating, a P0940 specifically refers to the electrical circuit's input. Overheating itself might cause the sensor to read high, but 'high input' usually points to a sensor or wiring issue, not necessarily the fluid level directly causing the electrical anomaly.
Do I need special tools to replace the sensor?
Replacement often requires basic hand tools like wrenches or sockets. You may need to drain some transmission fluid if the sensor is located in the pan or side of the transmission, so a drain pan will be necessary.