TEST DESCRIPTION:
The Manifold Air Temperature (MAT) 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 will detect a high signal voltage. If the air warms, the sensor resistance becomes less, and the voltage drops.

NOTE: A "Scan" tool may not be used to diagnose this fault, due to the ECM transmitting "Default" (Substitute) values when the fault is present.

MAT Sensor
Temperature vs Resistance (Approx)
Deg. F	Deg. C	Ohms
210	100	185
160	70	450
100	38	1,800
70	20	3,400
40	4	7,500
20	-7	13,500
0	-18	25,000
-40	-40	100,700

TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart. Code 25 will set if the engine is not experiencing turbocharger boost and the following conditions are met:

• Signal voltage indicates a manifold air temperature greater than 135'C (275'F).
• The above requirement is met for at least 30 seconds.

Due to the conditions necessary to set a Code 25, the "Check Engine" light will only stay "ON" while the fault is present.

1. A "Scan" tool may not be used to diagnose this fault due to the ECM transmitting "default" (substitute) values while the fault is present. If voltage is above 4 volts, the ECM and wiring are OK.
2. If the resistance is less than 100 ohms, replace the sensor.