With TBI

Code 23 Chart:

Wiring Diagram For MAT/IAT Sensor:

CIRCUIT DESCRIPTION:

The Manifold Air Temperature (MAT) sensor uses a thermistor that controls the signal voltage to the ECM. The ECM applies a reference voltage (4.0-6.0 volts) on circuit 472 to the sensor. When the air is cold, the sensor (thermistor) resistance is high, therefore the ECM will sense a high signal voltage. If the air is warm, the sensor resistance is low, therefore the ECM will see a low voltage.


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

1. This checks to see if a Code 23 is the result of a hard code failure or an intermittent problem.
A Code 23 will set under the following conditions:
a. The engine has been running longer than two minutes.
b. The signal voltage indicates a MAT reading of less than -35°C (-31°F).
c. The engine NOT running, and MAT reading is less than -35°C (-31°F) while the coolant temperature is greater than -26°C (-15°F).

2. Simulates a Code 25. If the "Scan" tool displays a high temperature, the ECM and wiring are OK.

3. This step checks the continuity of circuits 472 and 469. If circuit 469 is open, there may be a Code 33 stored.


DIAGNOSTIC AIDS:

1. If the engine has been allowed to cool to ambient temperature, coolant and MAT temperatures may be checked with a "Scan" tool. The readings should be close to each other.
2. The "Temperature to Resistance Value" scale above can be used to test the coolant sensor at various temperature levels to evaluate the possibility of a "skewed" (mis-scaled) sensor. A "skewed" sensor could result in driveability complaints.
3. A Code 23 will result if circuits 472 or 469 becomes open. If Code 23 is intermittent, see DIAGNOSIS BY SYMPTOM.