Code 23 - Manifold Air Temperature Sensor (MAT) Circuit

(Low temperature indicated)

Circuit Description

The MAT sensor uses a thermistor to control the signal voltage to the ECM. The ECM applies a voltage (about 4-6 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 see a low voltage

Problem Description:

Code 23 will set if:

• A signal voltage indicates a manifold air temperature below -25 C (-13 F) for 3 seconds.

• Time since engine start is 1 minute.

  OR

• Engine not running.

• MAT temperature less than -25 C (-13 F).

• No Code 15.

• Coolant temperature reading greater than -26 C (-15 F).

Diagnostic Aids:

A Scantool reads temperature of the air entering the engine and should read close to ambient air temperature when engine is cold, and rises as underhood temperature increases.

Inspect CKT's 472 & 469 for intermittent opens. If Code 33 is also set carefully inspect CKT 469.

"Scan" Diagnostics:

Does scantool display MAT -30 C or colder?

NO YES