With Speed Density Update

Manifold Air Temperature (MAT) Sensor Circuit:

CIRCUIT DESCRIPTION
The Manifold Air Temperature (MAT) sensor uses a thermistor to control the signal voltage to the Electronic Control Module (ECM). The ECM applies a voltage (about 5 volts) on circuit 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. The MAT sensor is part of the Mass Air Flow (MAF) sensor assembly.

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

1. Code 25 will set if:
^ A signal voltage indicates a manifold air temperature greater than 135° C (293° F) for 1 second.
^ A vehicle speed is present.

NOTE: Due to the conditions necessary to set a Code 25 the "Service Engine Soon" light will remain "ON" while the signal is low and vehicle speed is present.

DIAGNOSTIC AIDS
A "SCAN" tool 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. A short to ground in circuit 472 will result in a Code 25. The "Temperature to Resistance Values" scale at the right may be used to test the MAT sensor at various temperature levels to evaluate the possibility of a "shifted" (mis-scaled) sensor. A "slewed" sensor could result in poor driveability complaints.

Refer to Diagnosis by Symptom / Intermittents. - Intermittent Malfunctions