CIRCUIT DESCRIPTION:
The Manifold Air Temperature (MAT) sensor uses a thermistor to control the signal voltage to the ECM The ECM applies a voltage (about 5 volts) on CKT 472 to the sensor. When the air is cold the sensor (thermistor) resistance is high, therefore the ECM will see a high signal voltage. If the air is warm, the sensor resistance is l( therefore, the ECM will detect a low voltage.

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 23 will set if:

• A signal voltage indicates a manifold air temperature below - 30'C (-22'F).
• Boost conditions have been present for longer than 10 seconds.

Due to the conditions necessary to set a Code 23, 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. A Code 23 will set due to an open sensor, wire, or connection. This test will determine if the wiring and ECM are OK.
2. If the resistance is greater than 25,000 ohms, replace the sensor.