Powertrain Controls - ECM/PCM

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 low, therefore, the ECM will detect a low voltage.

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.