What does it mean?
The ambient air temperature sensor, usually mounted at the front of the vehicle behind the grille or in a mirror housing, tells the module how warm the outside air is. After a long soak the engine has given up its heat, so coolant temperature sensor 1 should read close to that ambient value. This code is stored when the two disagree by more than the calibrated amount at the first key on, meaning one of them is reporting incorrectly even though both are within their electrical range. Because the ambient value is often shared over the vehicle network and used by the climate system as well, the fault can show up as a wrong outside temperature display as much as an engine complaint.
Symptoms
- MIL on, usually noticed after the vehicle has stood overnight
- Outside temperature display clearly wrong or slow to update
- Climate control behaving oddly, blowing warmer or colder than requested
- Rough or long cold start and poor fuelling during warm up
- Cooling fan running earlier or later than expected
- Higher fuel consumption on short journeys
Likely causes
- Ambient air temperature sensor heat soaked by radiator or road heat because of a blocked grille or a poor mounting position
- Ambient sensor damaged or its harness broken after front end impact or bumper work
- Coolant temperature sensor 1 drifted so it reads warmer or cooler than the actual engine
- Low coolant level or an air pocket leaving the coolant sensor tip uncovered
- High resistance in either sensor circuit shifting the reading
- Incorrect replacement sensor with a different resistance curve
- A network communication fault so the module receives a stale or default ambient value from another module
How to fix it
- After a full cold soak, read coolant temperature and ambient temperature with the key on before starting and compare both with a real thermometer reading
- Check where the ambient sensor is mounted for blocked airflow, road debris and previous accident repair
- Measure each sensor resistance or signal voltage against its temperature curve at a known temperature
- Verify the coolant level and bleed any air, since a sensor tip in air reads wrong
- Test both circuits for voltage drop on the ground and reference sides and inspect the connectors
- Confirm that the ambient value the engine module uses matches what the source module transmits, to rule out a data fault
- Replace the sensor proven to be wrong, then repeat the comparison after another cold soak