For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provides an overview of each diagnostic category.

### DTC Descriptors

DTC P0072: Ambient Air Temperature (AAT) Sensor Circuit Low Voltage
DTC P0073: Ambient Air Temperature (AAT) Sensor Circuit High Voltage
DTC P0074: Ambient Air Temperature (AAT) Sensor Circuit Intermittent

### Diagnostic Fault Information

Circuit | Short to Ground | High Resistance | Open | Short to Voltage | Signal Performance
--- | --- | --- | --- | --- | ---
AAT Sensor Signal | P0072 | P0073 | P0073 | P0073 | P009A, P0074
Low Reference | -- | P0073 | P0073 | P0073 | P0074

### Typical Scan Tool Data

#### Ambient Temperature

Circuit | Short to Ground | Open | Short to Voltage
--- | --- | --- | ---
Operating Conditions: The ignition is ON, or the engine is running Parameter Normal Range: Varies with ambient temperature
AAT Sensor Signal | 90°C (194°F) | -40°C (-40°F) | -40°C (-40°F)
Low Reference | -- | -40°C (-40°F) | -40°C (-40°F)

### Circuit/System Description

The ambient air temperature (AAT) sensor system used on this vehicle consists of two separate sensors mounted in the grille area. The AAT sensors are variable resistors that measure the temperature of the air as it enters the grille opening. This system is uniquely wired as explained by the following:

• | The driver side mounted AAT sensor reports information to the logic module. The logic module supplies 5.8 volts to the sensor signal circuit and a ground for the low reference circuit.

• | The passenger side mounted AAT sensor reports information to the engine control module (ECM). However the circuits for this sensor are first routed and connected to the underhood fuse block. The internal circuitry of the underhood fuse block is a direct pass-through for the circuits that are then routed to the ECM. The ECM supplies 5 volts to the passenger side mounted AAT sensor signal circuit and a ground for the low reference circuit.

### Conditions for Running the DTC

#### P0072

• | The ignition is ON or the engine is running.

• | The ignition voltage is greater than 9 volts.

• | This DTC runs continuously within the enabling conditions.

#### P0073

• | The ignition is ON or the engine is running.

• | The ignition voltage is greater than 9 volts.

• | This DTC runs continuously within the enabling conditions.

#### P0074

• | DTCs P009A, P0072, or P0073 is not set.

• | The engine is running for greater than 30 seconds.

• | The ignition voltage is greater than 9 volts.

• | The vehicle speed is greater than 21 km/h (13 mph).

• | This DTC runs continuously within the enabling conditions.

### Conditions for Setting the DTC

#### P0072

The ECM detects that the AAT sensor signal is less than 0.5 volts, which is warmer than 90°C (194°F) for greater than 2 seconds.

#### P0073

The ECM detects that the AAT sensor signal is more than 4.90 volts, which is colder than -39°C (-38°F) for greater than 2 seconds.

#### P0074

The ECM detects that the vehicle is moving and the measured AAT is not within a calibrated range of the modeled AAT for greater than 2 seconds.

### Action Taken When the DTC Sets

DTCs P0072, P0073, P0074 are Type B DTCs.

### Conditions for Clearing the MIL/DTC

DTCs P0072, P0073, P0074 are Type B DTCs.

### Diagnostic Aids

• | The AAT sensors respond quickly to temperature changes, however the ECM processes an extremely large number of samples before it updates the information that is sent to the scan tool. An update to the scan tool Ambient Temperature parameter may not occur until the next drive cycle.

• | It is not possible to swing the scan tool Ambient Temperature parameter warm or cold as is usually done with traditional methods of testing when servicing the intake air temperature (IAT) sensor or the engine coolant temperature (ECT) sensor.

• | The resistance of the AAT sensors respond quickly to temperature changes. These changes can be monitored with a DMM. Simply placing the sensor in a refrigerator or freezer and then removing it after several minutes will induce a rapid change in the resistance measurement as it warms up.

• | The AAT sensors are identical and can be exchanged with each other for testing purposes.

### Reference Information

#### Schematic Reference

Engine Controls Schematics

#### Connector End View Reference

Component Connector End Views

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

#### Scan Tool Reference

Control Module References for scan tool information.

### Circuit/System Verification

• | If the ignition has been OFF for 8 hours or greater, the Ambient Temperature, the IAT and the ECT should be within 9°C (16°F) of each other. Ignition ON, observe the scan tool Ambient Temperature, IAT Sensor and the ECT Sensor parameters. Compare those sensor parameters to each other to determine if the condition is current.

• | Drive the vehicle under various operating conditions. Observe the scan tool Ambient Temperature parameter. The reading should correlate closely to the current ambient temperature.

• | Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

### Circuit/System Testing

1. Verify that debris is not blocking the airflow through the front of the grille area.
2. Verify that the port of the sensor is free of debris and contamination.
3. Ignition OFF for 90 seconds, disconnect the harness connector at the passenger side AAT sensor.
4. Test for less than 5 ohms between the low reference circuit and ground.

⇒ | If greater than the specified range, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the ECM.

5. Ignition ON, test for 4.8-5.2 volts between the AAT sensor signal circuit and ground.

⇒ | If less than the specified range, test the signal circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the ECM.

⇒ | If greater than the specified range, test the signal circuit for a short to voltage. If the circuit tests normal, replace the ECM.

6. If the circuits test normal, test or replace the AAT sensor.

### Component Testing

Allow the sensor to rest on a workbench away from sunlight and any direct heat or cold source. Connect the DMM to the sensor, and measure the resistance at an ambient temperature of 22-23°C (72-73°F). The resistance should measure between 4.9-5.4K ohms.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for ECM replacement, setup, and programming