Powertrain Controls - ECM/PCM

Diagnostic Chart:

IAT Sensor Circuit:

CIRCUIT DESCRIPTION
The Intake Air Temperature (IAT) sensor is a thermistor (a variable resistor that changes along with outside air temperature) in series with a fixed resistor in the Engine Control Module (ECM). The ECM applies 5 volts to the sensor. The ECM monitors the voltage across the IAT sensor and converts it into a temperature reading. When the outside air temperature is cold the IAT sensor resistance is high, and when the outside air temperature is warm the IAT sensor resistance is low. Therefore, when the air temperature is cold, the ECM will receive a high voltage input and when the air is warm, the ECM will receive a low voltage input.

Diagnostic Trouble Code (DTC) 25 will set if the following condition is met for at least 1.6 seconds: Voltage input at the ECM indicates an air temperature above 136°C (276°F).

DTC CHART TEST DESCRIPTION
The number below refers to the circled number on the diagnostic chart.

1. This test simulates DTC 23. If the ECM stores DTC 23, the ECM and its wiring are OK and the IAT sensor is faulty. If DTC 25 resets, the IAT sensor is OK, and the wiring to the ECM or the ECM is faulty.

DIAGNOSTIC AIDS
When replacing the IAT sensor, the "Temperature to Resistance Value" scale on the diagnostic chart may be used to test the sensor at various temperature levels to evaluate the possibility of a shifted (mis-scaled) sensor. A shifted sensor could result in poor driveability complaints.

Refer to Computer and Control Systems / Intake Air Temperature Sensor for additional information. Air Temperature Sensor ( Ambient / Intake )

INTERMITTENT MALFUNCTIONS
An intermittent malfunction may be caused by a poor connection, rubbed through wire insulation, or a wire broken inside the insulation. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals and poor terminal-to-wire connections before component replacement.