P009A: Intake Air Temperature/Ambient Air Temperature Correlation
What to do
Leaving the car overnight genuinely helps here. This fault is found from stone cold, when both sensors should read the same as the outside air — a warm engine tells the garage much less, because the two legitimately differ once running.
Mention any recent work on the intake, the front bumper or the radiator area. A sensor moved or refitted slightly differently reads the wrong air and produces this with nothing broken.
Say if the outside temperature shown on the dashboard looks wrong, particularly if it reads high when you are stationary in traffic and settles once moving. That points at the ambient sensor.
The car will usually drive normally, but the mixture and emissions calculations use these readings, so it is worth fixing before an MOT.
What this code means
The intake air temperature and the ambient air temperature disagree by more than the module allows.
These two sensors measure air that should be the same air, at least some of the time — and that is the whole basis of the check. The ambient sensor sits in the airflow at the front of the car; the intake sensor sits in the induction tract. With the engine off and cold, they must agree.
Running, they legitimately differ, and knowing why prevents a false diagnosis. Intake air is warmed by the engine bay, and on a turbocharged engine it is heated hard by compression — so an intake reading well above ambient at load is correct rather than faulty. The module accounts for that; the check is tightest when the vehicle has been standing.
Which makes the cold soak the decisive test, as with every correlation code. After the vehicle has stood overnight, every temperature sensor on it should read close to ambient and close to each other. The one that disagrees then has identified itself, and no road test is needed.
Position causes false readings more often than failure does. An ambient sensor mounted behind the grille sits in radiator heat when the car is stationary, which is why many vehicles freeze its displayed value below a certain speed — and an intake sensor moved or badly refitted after intake work reads the engine bay rather than the incoming air.
Ask what has been disturbed recently before condemning either sensor.
Likely causes
Start here
Less common
- Sensor incorrectly positioned after repair · easy
- High resistance in sensor wiring · moderate
- Sensor heat soak from poor mounting or airflow · moderate
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
Be the first to write this up
Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
Sign in or create an account to add what you found about P009A.