Sensor Troubleshooting: Intake Air TempProbing bogus incoming-air readings
The Check Engine/Service Engine Soon light strikes fear in the stomachs of even the most jaded drivers. Humans seem to have an inherent fear of electricity, and we assume that any computer-related malfunction will be expensive. However, many at-home tests can help vehicle owners narrow down the problem and decide whether or not to tackle it themselves.
The Check Engine light often accompanies a noticeable deterioration in vehicle performance. Fortunately, the car's computer normally stores a "trouble code" when the Check Engine light illuminates. Accessing the code varies by vehicle make and model year. Affordable "scan" tools are available at most parts stores for many popular vehicles. These attach to the vehicle's ALDL (Assembly Line Data Link) diagnostic connector to produce the trouble code. Explanations for the codes are in the scan tool's instruction manual as well as in service manuals for the particular vehicle.
The procedure shown here is on an '80's GM car. For GM vehicles that have 12-pin ALDLs, such as this one, no scanner is necessary. Simply run a jumper wire or even a paper clip between the two upper-right terminals. Just be sure to disconnect the jumper wire/scan tool before starting the vehicle.
If the trouble code corresponds to a MAT (Manifold Air Temperature) or an IAT (Intake Air Temperature) sensor, the fix is doable for most at-home car hackers. Functionally, this temperature-probe sensor helps the computer regulate exhaust-gas recirculation based on incoming-air temp. The sensor is either located in the air-cleaner housing (as it is here) or air duct. Removing the IAT/MAT sensor is usually straightforward.
Sensor testing requires an ohmmeter and a household hair drier. First, test the MAT's resistance when cold. Then, warm up the sensor with the hair drier and take another reading. Check a service manual for the acceptable ohm range.
IAT/MAT sensor replacement is the reverse of its removal. If the Check Engine light is still on and the computer continues to give the IAT/MAT trouble code, consider disconnecting the car's negative battery cable for five minutes or more to "clear" the computer's memory. (Always consult the service manual before disabling the battery about potential problems with alarm systems and such.)
If the problem still isn't solved, rescan the computer for multiple trouble codes. Failing this, take the vehicle in for service, armed with the knowledge that the IAT/MAT sensor itself is indeed functional.
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