Powertrain Controls - ECM/PCM

Code 25 Diagnostic Chart:

TPS, CTS, & IAT Sensors (Circuit Diagram):

CIRCUIT DESCRIPTION:
The IAT sensor is a thermistor. The PCM applies a voltage (about 5 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:

^ Signal voltage indicates a intake air temperature greater than 103°C (218°F).
^ Vehicle speed is greater than 35 mph.
^ Both of the above requirements are met for at least 5 seconds.

TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.
1. A "Scan" tool may be used to diagnose this fault since the PCM transmits "actual" values when the fault is present. A grounded circuit will cause the scan tool to display a temperature of approximately 147°C (297°F).
2. If "SCAN" tool displays temperature of -40 with the sensor disconnected, then the PCM and wiring are OK. Check sensor resistance. If the resistance is less than 185 ohms, replace the sensor.

DIAGNOSTIC AIDS
^ An intermittent fault may be caused by wire insulation burned or worn through (wire intermittently shorted to ground.)

Check for:
^ Poor connection or damaged wiring 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, monitor signal voltage while moving related connectors and wiring harness with the engine running at normal operating temperature. If the failure is induced, the display will change (show very high temperature). This may help to isolate the location of the malfunction.