Technical Description of OBD-II Trouble Code
Correlation Between Intake Air Temperature and Ambient Air Temperature
What does that signify?
This generic powertrain diagnostic trouble code (DTC) is commonly found in many OBD-II vehicles, including those from Mercedes-Benz, Jeep, Mazda, Ford, and others.
If you have a service engine soon lamp accompanied by a code P009A, it means that the powertrain control module (PCM) has detected a discrepancy in the signals between the intake air temperature (IAT) sensor and the ambient air temperature sensor. The PCM compares the IAT and ambient air temperature to ensure that there are no obstructions affecting the airflow to the engine intake.
IAT sensors are typically composed of a thermal resistor that extends from a plastic housing on a two-wire pedestal. The sensor is inserted into the air intake pipe or air filter housing. Another design integrates the IAT sensor inside the mass air flow (MAF) sensor housing. The location of the IAT sensor varies, so it is important to check the specifications for the specific vehicle.
The thermal resistor is positioned to allow intake air to flow across it. The sensor housing is designed to be inserted into its mounting point through a rubber grommet. As the intake air temperature increases, the resistance in the IAT resistor decreases, causing the circuit voltage to move towards the reference maximum. When the air is cooler, the IAT sensor resistance increases, resulting in decreased circuit voltage. The PCM interprets these voltage variations as changes in intake air temperature.
The ambient air temperature sensor functions similarly to the IAT sensor and is usually located near the grill area.
A code P009A will be stored and the malfunction indicator lamp (MIL) may illuminate if the PCM detects voltage signals from the IAT sensor and the ambient air temperature sensor that differ by more than the maximum allowable amount for a certain period of time. In some cases, MIL illumination may require multiple ignition cycles with a failure.
What is the severity of this DTC?
A stored code P009A should be considered severe as the input from the IAT sensor is crucial for fuel delivery.
What are some of the symptoms of the code?
Symptoms of a P009A engine code may include:
What are some of the common causes of the code?
Possible causes for this engine code may include:
What are some P009A troubleshooting steps?
Prior to diagnosing a P009A, you will need an infrared thermometer with a laser pointer, a diagnostic scanner, a digital volt/ohmmeter (DVOM), and reliable vehicle information.
If a stored IAT sensor code is present, inspect the air filter element to ensure it is clean and properly inserted into the housing. Additionally, visually inspect the wiring and connectors of the IAT sensor and ambient air temperature sensor system if the air filter element appears to be functioning correctly.
Next, connect the scanner to the vehicle diagnostic port and retrieve all stored codes and freeze frame data. It is recommended to write down this information as it may be helpful during the diagnostic process. Clear the codes and test-drive the vehicle to see if the P009A code is reset. Refer to the vehicle information source for wiring diagrams, connector pin out charts, component testing specifications, and connector views specific to the vehicle being diagnosed. When testing individual circuits and sensors, remember to unplug the PCM (and all related controllers) to prevent controller damage and test system circuits for resistance and continuity with the DVOM.
Testing the IAT and Ambient Air Temperature Sensors:
Sensors that do not comply with the testing specifications should be considered defective.
Check for Reference Voltage and Ground:
Test IAT and Ambient Air Temperature Sensor Signal Circuits:
If both the IAT and ambient air temperature sensor signal circuits reflect the appropriate amount of voltage
Exhaust all other possibilities and consider PCM failure (or a PCM programming error) only if all IAT and ambient air temperature sensors and circuits are within specifications.
Technical service bulletins (TSB) that match the vehicle, symptoms, and stored codes may assist in the diagnosis.
