What does it mean?
This is a plausibility fault, not a wiring fault. The signal from coolant temperature sensor 3 is electrically valid, but the value it reports does not make sense to the module. Typical triggers are a reading that disagrees with the other coolant and air temperature sensors after a long cold soak, a value that hardly moves while the engine warms up, or one that sits stuck at a fixed number. Because the module builds fuelling, fan control and monitor entry conditions on this value, an inaccurate but believable reading is more troublesome than an obviously dead sensor. The cause can be the sensor drifting with age, or a genuine cooling problem in the loop the sensor is watching.
Symptoms
- Long warm-up times and a heater that stays lukewarm, especially in cold weather.
- Cooling fans that run at the wrong time, either far too early or too late.
- Temperature gauge that reads lower or higher than the engine actually feels to the touch.
- MIL (check engine lamp) on after a couple of drive cycles, often with the vehicle running otherwise normally.
- Higher fuel consumption because the module keeps the engine in a warm-up enrichment strategy.
- Readiness monitors that never complete, since they need a valid coolant temperature to start.
Likely causes
- Aged or drifted thermistor in sensor 3, reading a consistent offset from the true coolant temperature.
- Sensor stuck at one value because of internal contamination or corrosion at its tip.
- Low coolant level or trapped air, so the sensor is surrounded by vapour instead of liquid and reports a false temperature.
- Thermostat stuck open or opening early, keeping that part of the circuit cooler than the model expects.
- Poor flow through the loop the sensor monitors, from a blocked hose, a failing auxiliary pump or a clogged radiator.
- Added resistance in the signal or ground wiring, shifting the reading by a constant amount without setting a circuit code.
- Wrong sensor part fitted during previous work, with a different resistance curve to the one the module expects.
How to fix it
- After an overnight cold soak, compare all coolant, oil and intake air temperature readings; they should agree closely, and any channel more than a few degrees away is suspect.
- Graph the value during a warm-up from cold and look for a smooth, continuous rise rather than a flat line or steps.
- Check coolant level and bleed any air trapped in the circuit, since a sensor sitting in an air pocket produces exactly this fault.
- Compare the reading with an infrared measurement taken on the housing or hose next to the sensor.
- Verify thermostat operation and flow through the loop; a thermostat that opens early keeps the circuit permanently cool.
- Perform voltage drop tests on the signal and ground wires to rule out added resistance, then measure sensor resistance against the specification table.
- Renew the sensor with the correct part number if it is out of tolerance, then clear the code and confirm the warm-up curve on a road test.