What does it mean?
Engines that use two coolant temperature sensors normally place the second one away from the first, for example at the radiator outlet or in a separate cooling circuit, so the module can see the temperature difference across the system and control the fans and the thermostat accordingly. While the engine runs the two sensors legitimately read differently, but after a long soak everything is at ambient temperature. This code is stored when coolant temperature sensor 2 disagrees with the ambient air temperature by more than the calibrated amount at the first key on. It therefore points to a drifted sensor 2, a wrong ambient value or coolant that is not actually surrounding the sensor tip.
Symptoms
- MIL on, generally set at the first start after a long stand
- Cooling fans running at the wrong time, either too early or not at all
- Engine temperature gauge behaviour that does not match cabin heat output
- Slow warm up or an engine that runs cooler than normal
- Higher fuel use and a rough cold start
- Outside temperature display that looks wrong when the ambient sensor is the faulty one
Likely causes
- Coolant temperature sensor 2 drifted so it reads warm or cold without leaving its electrical range
- Ambient air temperature sensor heat soaked, damaged or blocked by debris at the front of the vehicle
- Low coolant level or trapped air leaving sensor 2 out of the liquid, which is more likely at the radiator outlet
- High resistance in the signal or ground circuit of either sensor
- Incorrect replacement sensor with a different resistance curve
- Corroded connector or a damaged pin at one of the module inputs
- A network fault delivering a default or stale ambient temperature to the engine module
How to fix it
- After a full cold soak read coolant temperature 2 and ambient temperature with the key on, and compare both with an accurate thermometer
- Compare coolant sensor 1 and 2 with each other in the same condition; all three values should be close before the engine starts
- Measure sensor resistance or signal voltage against its temperature specification
- Check coolant level and bleed the cooling system, paying attention to the radiator outlet area where air collects
- Inspect the ambient sensor location for blocked airflow, damage and previous front end repair
- Test both circuits for voltage drop and inspect connectors for corrosion and spread terminals
- Replace the sensor proven to be wrong and repeat the cold soak comparison to confirm the repair