Without Speed Density Update

Chart - Code 23 Manifold Air Temperature (Signal Voltage High):

Wiring Diagram for MAT & MAF Sensors.:

CODE 23 MANIFOLD AIR TEMP. (MAT) SENSOR (LOW 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) 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 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.
1. Code 23 will set if:
A signal voltage indicates a manifold air
temperature below -35 degrees C (-31 degrees F).
Time since engine start is 4 minutes or longer.
No VSS.
2. A Code 23 will set, due to an open sensor, wire, or connection. This test will determine if the wiring and ECM are OK.
3. This will determine if the signal CKT 472 or the 5 volt return CKT 452 is open.


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 faulty connection, or an open in CKT 472 or 452 will result in a Code 23. Codes 23 and 34 stored at the same time, could be the result of an open CKT 452 which would also turn the temperature warning indicator "ON". CKT 452 is routed through a mini harness. A faulty connection could result in intermittent failures. 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