DTC 23

Fig. 033 Code 23 Manifold Air Temperature (MAT) Sensor:

Fig. 033a Code 23 Manifold Air Temperature (MAT) Sensor:

CIRCUIT DESCRIPTION:
The Manifold 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, so the ECM sees a high signal voltage. As the air warms, the sensor resistance becomes less and the voltage drops.

TEST DESCRIPTION: Numbers below refer to circled numbers on Diagnostic Charts.

1. This step checks to see if Code 23 is the result of a hard failure or, an intermittent condition.
A Code 23 will set if:
^ Signal voltage indicates a MAT temperature less than -30 degrees C.
^ Engine is running for longer than 58 seconds.
2. This test simulates conditions for a Code 25. If the "Scan" tool displays a high temperature, the ECM and wiring are OK.
3. This step checks continuity of CKTs 472 and 469. If CKT 469 is open there may also be a Code 33.

DIAGNOSTIC AIDS:
If the engine has been allowed to cool to an ambient temperature (overnight), coolant and MAT temperatures may be checked with a "Scan" tool and should read close to each other. A Code 23 will result is CKTs 472 or 469 become open. If Code 23 is intermittent, refer to DIAGNOSIS BY SYMPTOM.