Powertrain Controls - ECM/PCM

Code 25 Diagnostic Chart:

Intake Air Temperature (IAT) Sensor Wiring Schematic:

CIRCUIT DESCRIPTION
The Intake Air Temperature (IAT) sensor uses a thermistor to control the signal voltage to the Powertrain Control Module (PCM). The PCM applies a voltage (about 5 volts) on CKT 472 to the sensor. When intake air is cold, the sensor (thermistor) resistance is high, therefore, the PCM terminal "PC9" voltage is high. If the air is warm, the sensor resistance is low, therefore the PCM terminal "PC9" voltage will be low.

DTC 25 WILL SET WHEN
^ Signal voltage indicates a intake air temperature greater than about 151°C (304°F).
^ Vehicle speed is greater than 15 mph.
^ Engine run time greater than 320 seconds.

ACTION TAKEN (PCM WILL DEFAULT TO)
Malfunction Indicator Lamp (MIL) illuminates. PCM will default to 60°C (140°F) for intake air temperature.
NOTE: Tech 1 scan tool will not display defaulted value.

TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. This step checks if DTC 25 was set as an intermittent or a hard condition

DIAGNOSTIC AIDS
^ The "Temperature vs. Resistance Value" scale, in the diagnostic chart, may be used to test the IAT sensor at various temperature levels to evaluate the possibility of a "skewed" (mis-scaled) sensor. A "skewed" sensor could result in poor driveability complaints.
^ Refer to "DIAGNOSIS BY SYMPTOM/INTERMITTENTS" Testing and Inspection