Pontiac Fiero 1988 Service Manual page 619

6E3-A-36 DRIVABILITY
A N D
E M I S S I O N S - F U E L
INJECTION (PORT)
5-8-87
* 5 S 1 9 1 1 6E
CODE 23
MANIFOLD AIR TEMPERATURE (MAT) SENSOR CIRCUIT
(LOW TEMPERATURE INDICATED)
2.8L "P" SERIES (PORT)
Circuit Description:
The MAT sensor uses a thermistor to control the signal voltage to the ECM. The ECM applies a voltage
(about 4-6 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.
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 -25°C (-13°F) for 3
seconds.
o Time since engine start is 1 minute
or
• Engine not running
• MAT temperature less than - 2 5 ° C (-13°F)
• No Code 15
• Coolant temperature reading greater than -
2 6 ° C (-15°F)
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
5V return CKT 469 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.
Inspect CKTS 472 & 469 for intermittent opens. If
Code 33 is also set carefully inspect CKT 469.
Refer to "Intermittents" in Section "B".