Without Speed Density Update

Chart - Code 25 Manifold Air Temperature (Signal Voltage Low):

Wiring Diagram for MAT & MAF Sensors.:

CODE 25 MANIFOLD AIR TEMP. (MAT) SENSOR (HIGH TEMP. INDICATED)


Circuit Description:
The 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) resistant is high therefore the ECM will see a high signal voltage. If the air is warm the sensor resistance is low ECM will see a low voltage. The MAT sensor is part of the MAF sensor assembly so the ECM can accurately compensate the air now reading based on temperature.


The MAT sensor is part of the MAF sensor assembly so the ECM can accurately compensate the air flow reading based on temperature.


Test Description: Numbers below refer to circled numbers on the diagnostic chart. Code 25 will set if-
Signal voltage indicates a manifold air temperature greater than 135 degrees C (293 degrees F) for 3 seconds.
Time since engine start is 8 minutes or longer.
A vehicle speed over 3 mph is present.
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 CKT 472 will result in a Code 25. The "Temperature to Resistance Values" scale may be used to test the MAT sensor at various temperature levels to evaluate the possibility of a "shifted" (mis-scaled) sensor. a "slowed" sensor could result in poor driveability complaints. Refer to "Intermittents" in "DIAGNOSIS BY SYMPTOM - NO TROUBLE CODE STORED".





*** NOTE: Before replacing MAF sensor, see TSB #886E31 DATED JUNE, 1990