Engine

P009A — Intake Air / Ambient Air Temperature Correlation Fault

Severity: low Safe to drive

What P009A means

The engine control module compares the intake air temperature (IAT) sensor with the ambient air temperature (AAT) sensor. After the engine has sat long enough to cool down, the two readings should be close. If they differ too much, or one sensor appears stuck, the module sets P009A and turns on the check engine light.

Symptoms

  • Check engine light on
  • Slightly worse fuel economy
  • Hesitation or sluggish acceleration
  • Sometimes no noticeable symptoms
  • Odd outside temperature readout on the dash

Possible causes

  1. Failed intake air temperature (IAT) sensor
  2. Failed ambient air temperature (AAT) sensor
  3. Corroded or loose sensor connectors
  4. Open, shorted or chafed wiring
  5. IAT sensor heat-soaked by nearby components or installed incorrectly
  6. Rarely, ECM software fault or failure

How to diagnose

  1. Scan for all codes and review freeze frame data, noting any other temperature sensor codes
  2. Let the engine cool completely, then compare IAT and AAT in live data: they should nearly match
  3. Compare both values to the actual air temperature to find which sensor is wrong
  4. Inspect both sensor connectors and wiring for corrosion, moisture, chafing and poor pin fit
  5. Measure sensor resistance with a multimeter and compare to the spec for your vehicle
  6. Check reference voltage and ground at the sensor connector
  7. Unplug a sensor to see if the reading changes sharply, which confirms the wiring is intact
  8. Clear the code and road test after repair

Common fixes

  • Replace the intake air temperature sensor
  • Replace the ambient air temperature sensor
  • Clean terminals or replace the connector, and repair damaged wiring
  • Update ECM software or replace the module if confirmed faulty

Questions and answers

What does P009A mean?

The ECM sees that the intake air temperature and ambient air temperature sensors are giving readings that don't agree. Most often one of the sensors or its wiring is at fault.

Can I drive with P009A?

Usually yes, but don't put off diagnosis for long. You may see higher fuel consumption and a small loss of responsiveness.

How do I tell which sensor is bad?

Let the engine cool, then compare both sensors' live data with the real air temperature. The one that is far off is the likely culprit.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.