What does it mean?
This is a rationality fault on the second engine oil temperature sensor rather than an electrical one. The module compares oil temperature B with the other temperature inputs it trusts, mainly coolant temperature, intake air temperature and where fitted oil temperature A, and it also expects the value to rise at a predictable rate after a cold start and to settle in a normal band once the engine is at working temperature.
When the reading is stuck, warms far too slowly, or sits outside the allowed spread from those references, the module decides it is not usable even though the voltage is legal. This matters because oil viscosity corrections drive variable valve timing response, thermal management and the oil life calculation. In practice the fault splits between a drifted sensor and a genuine cooling or oil circulation problem that makes the oil behave unexpectedly.
Symptoms
- MIL (check engine lamp) on with an oil temperature rationality fault stored
- Oil temperature display, where fitted, that clearly disagrees with how hard the engine has been working
- Variable valve timing that responds sluggishly, sometimes with rough running or a slight loss of torque
- Thermal management or fan operation that seems inappropriate
- Oil life indicator not counting down as expected
- Little or no change in general driveability in mild cases
Likely causes
- Aged or drifted thermistor reading a consistent offset from true oil temperature
- High resistance in the signal or ground circuit shifting the calculated temperature without leaving the valid range
- Sensor not fully seated in its port or sealed incorrectly, so it does not sit in the oil flow
- Wrong sensor part fitted during a previous repair, with a different resistance curve
- Engine thermostat stuck open or an oil cooler thermostat fault, so the oil genuinely never reaches normal temperature
- Low oil level or a sludged sensor tip slowing the response
- One of the reference sensors used in the comparison being faulty itself
How to fix it
- After an overnight soak, read oil temperature B, coolant temperature and intake air temperature together; on a cold engine they should agree closely and the outlier is the suspect
- Warm the engine fully and watch how oil temperature B rises relative to coolant; a value that lags badly points at a thermostat or oil cooler issue rather than the sensor
- Check for other stored temperature codes, since a faulty reference sensor can trigger this comparison
- Measure the sensor resistance at a known temperature and compare it with the specification to expose drift or a wrong part
- Voltage drop test the signal and ground circuits, as corroded terminals can shift the reading by several degrees
- Verify oil level and condition, and confirm the sensor is correctly seated in its port
- Replace oil temperature sensor B when its resistance is out of specification and both the wiring and the cooling system have tested good