Powertrain Controls - ECM/PCM

Code 25:

CTS Circuit:

CIRCUIT DESCRIPTION:

The Intake Air Temperature (IAT) (previously MAT) sensor is a thermistor. The PCM applies a voltage (about 5.0 volts) on CKT 472 to the sensor. When air is cold, the sensor (thermistor) resistance is high, therefore, the PCM will measure a high signal voltage. If the air is warm, the sensor resistance is low, and the PCM will measure a low voltage.

Code 25 will set if:

^ Voltage at PCM terminal "BB7" indicates an intake air temperature greater than 103°C (218°F).
^ A vehicle speed is greater than 35.0 mph.
^ Both of the above requirements are met for at least 5.0 seconds.

TEST DESCRIPTION: Numbers below refer to circled numbers on the diagnostic chart.

1. Determines if conditions necessary to set code 25 exists.

2. With the IAT sensor disconnected, the Tech 1 should display an intake air temperature of about -40°C. If not, CKT 472 is grounded or the PCM is faulty.

DIAGNOSTIC AIDS:

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation.

Check For:

^ Poor Connection or Damaged Harness - Inspect PCM harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.

^ Intermittent Test - If connections and harness check OK, observe ED23/Tech 1 IAT display while moving related connectors and wiring harness with warm engine running at part throttle in "Closed Loop." If the failure is induced, the IAT display will change to over 120°C (250°F). This may help to isolate the location of the malfunction.