Powertrain Controls - ECM/PCM

Code 25 Chart:

Code 25 Wiring Diagram:

CIRCUIT DESCRIPTION:


The intake air temperature sensor uses a thermistor to control the signal voltage to the ECM. The ECM applies a voltage (4-6 volts) on CKT 472 to the sensor. When manifold air is cold, the sensor (thermistor) resistance is high, therefore, the ECM terminal "GF16" voltage is high. As the air warms, the sensor resistance becomes less, and the voltage drops. As the incoming air gets warmer, the sensor resistance decreases, causing ECM terminal "GF16" voltage to decrease.


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

1. Code 25 will set if:

^ Signal voltage indicates a manifold air temperature greater than about 159~C (318~F).
^ Vehicle speed is greater than 15 mph.



DIAGNOSTIC AIDS:


^ The "Temperature To Resistance Value" scale in chart above may be used to test the MAT sensor at various temperature levels to evaluate the possibility of a "slewed" (mis-scaled) sensor. This could result in poor driveability complaints.

^ - Intermittent Malfunctions