Pontiac Fiero 1988 Service Manual page 495

6E2-A-40 DRIVEABILITY AND EMISSIONS - FUEL INJECTION (TBI)
CODE 25
M A N I F O L D AIR T E M P E R A T U R E ( M A T ) S E N S O R C I R C U I T
(HIGH TEMPERATURE INDICATED)
2.5L " P " SERIES (TBI)
Circuit Description:
The manifold air temperature sensor uses a thermistor to control the signal voltage to the ECM. The ECM
applies a voltage (4-6 volts) 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. This check determines if the Code 25 is the result
of a hard failure or an intermittent condition.
A Code 25 will set if:
• A MAT temperature greater than 1 3 5 ° C is
detected for a time longer than 2 seconds.
Diagnostic Aids:
If the engine has been allowed to cool to an
ambient temperature (overnight), coolant and MAT
temperatures may be checked with a "Scan" tool and
should read close to each other.
A Code 25 will result if CKT 472 is shorted to
ground.
If Code 25 is intermittent, refer to Section "B".