Pontiac Fiero 1988 Service Manual page 623

6E3-A-40 DRIVAB1LITY AND EMISSIONS FUEL INJECTION (PORT)
5-8-87
5 19-87 *5S1911-6E
CODE 25
MANIFOLD AIR TEMPERATURE (MAT) SENSOR CIRCUIT
(HIGH TEMPERATURE INDICATED)
2.8L
f f
P " SERIES (PORT)
Circuit Description:
The manifold air temperature (MAT) sensor uses a thermistor to control the signal-voltage to the ECM.
The ECM applies a voltage (4-6) on CKT 472 to the sensor. When manifold air is cold, the sensor (thermistor)
resistance is high, therefore, the ECM will see a high signal voltage. As the air warms, the sensor resistance
becomes less, and the voltage drops.
Test Description:
Numbers below refer to circled
numbers on the diagnostic chart.
1. Code 25 will set if:
• Signal voltage indicates a manifold air
temperature greater than 1 3 5 ° C (275° F) for 2
seconds
• Time since engine start is 1 minute or longer
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 rise as
underhood temperature increases.
Check harness routing for possible short to ground
in CKT 472.
Refer to "Intermittents" in Section "B".