DTC 25

Intake Air Temperature (IAT) Sensor Circuit Diagram:

CIRCUIT DESCRIPTION
The IAT sensor is a thermistor with a negative temperature coefficient (as temperature increases, resistance decreases.) The PCM applies a 5 volt reference voltage through a resistor, to the sensor, on CKT 472, and then monitors the voltage in the circuit. When the air is cold, the sensors resistance is high, and the circuit voltage remains high (near reference voltage.) If the air is warm, sensor resistance decreases, causing drop in the measured voltage in the circuit. The PCM measures the voltage and calculates the temperature of the incoming air.

Code 25 will set when the following conditions exist:

^ Signal voltage indicates a intake air temperature greater than -135°C (293°F) for 0.2 seconds.
^ Vehicle speed sensor signal present, indicating vehicle is moving.

NOTE: Due to the conditions necessary for this code to set, the SES light will be "ON" only while conditions are present.

TEST DESCRIPTION

Code 25 Diagnostic Chart:

Numbers below refer to circled numbers on the diagnostic chart.
1. Determines if PCM is reading signal necessary for this code to set. If temperature display changes when sensor is disconnected, wiring and PCM are OK.

DIAGNOSTIC AIDS

^ A short to ground in circuit 472 will set a code 25.

^ See the Temperature/Resistance chart for testing sensor resistance values at different temperatures. A sensor that has incorrect resistance values for the temperature range (shifted sensor) can cause driveability problems.

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

^ 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.