Powertrain Controls - ECM/PCM

DTC 25:

IAT 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 ECM. The ECM applies 5.0 volts to the sensor. The ECM monitors the voltage across the IAT sensor and converts it into a temperature reading. When the air is cold, the sensor resistance is high, therefore, the ECM will see a high signal voltage. If the air is warm, the sensor resistance is low, therefore, the ECM will see a low voltage. DTC 25 will set if the following condition is met for at least 3 seconds: Voltage input at the ECM indicates an air temperature above 136°C (277°F).

TEST DESCRIPTION: Numbers below refer to circled numbers 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.