What does it mean?
After a vehicle has stood long enough for the engine to cool to the surrounding temperature, the coolant, the metal and the air in the intake are all at roughly the same temperature. The module uses that fact as a self test: at the first key on after a long soak it compares engine coolant temperature with intake air temperature and expects them to agree within a few degrees. This code is stored when the difference is larger than the calibrated window, which says one of the two sensors is reading wrong even though both are electrically inside their normal range. The comparison alone cannot say which sensor is lying, so the diagnosis has to establish that with a third reference.
Symptoms
- MIL on, often noticed the morning after the vehicle has stood overnight
- Long cranking or rough running on a cold start
- Rich or lean running during warm up, sometimes with a fuel smell
- Cooling fan switching on earlier or later than expected
- Heater output that does not match what the temperature gauge shows
- Higher fuel consumption, particularly on short trips
Likely causes
- Coolant temperature sensor drifted, so it reads warmer or cooler than reality without going out of range
- Intake air temperature sensor drifted, contaminated, or heat soaked because of poor airflow at its mounting point
- Low coolant level or an air pocket leaving the coolant sensor tip uncovered
- High resistance in a signal or ground circuit shifting one of the readings
- Thermostat stuck open, so the engine never behaves the way the warm up logic expects
- An incorrect replacement sensor with a different resistance curve
- Corroded module connector pin on one of the two inputs
How to fix it
- Let the vehicle stand until it is fully cold, then read both temperatures with the key on before starting; they should be close to each other and to the real ambient temperature
- Bring in a third reference: an infrared reading of the thermostat housing and the intake duct plus a measured ambient temperature
- Measure each sensor resistance and compare it with its temperature specification, or check the signal voltage against the same table
- Check the coolant level and bleed the system, since air around the sensor tip produces a false reading
- Test the ground and reference circuits for voltage drop, because a small shift can skew a reading without setting a range code
- Verify thermostat operation and warm up behaviour on a road test
- Replace whichever sensor is proven wrong with the correct specification part, then repeat the cold soak comparison