P1508

Circuit Description

This diagnostic test is used to determine if the Idle Air Control (IAC) system is maintaining the proper idle speed. The IAC system controls idle speed by allowing a certain amount of air to bypass the throttle body, the throttle plate is closed during idle, through the IAC valve. The PCM controlled IAC motor moves a pintle in and out of the IAC valve, thus varying the amount air entering the intake manifold. By varying the amount of air intake, different idle speeds can be achieved. When the pintle is in, the valve is closed and low idle speeds result. When the pintle is out, the valve is open and high idle speeds occur. If the engine idle speed is less than the desired idle speed and all test conditions are met, the PCM will cycle the IAC valve and expect to see a change in the MAF sensor reading. If no change occurs DTC P1508 is set.

Conditions for Running the DTC

^ DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0201, P0202, P0203, P0204, P0205, P0206, P0207, P0208 or P502 not set.
^ P1370, P1371, P1406 or P1441 not testing.
^ PCM not checking EGR operation.
^ Ignition voltage between 10.5 and 15 volts.
^ Barometric Pressure (BARO) at least 65 kPa.
^ ECT is between -20°C (-4°F) and 110°C (230°F).
^ Engine running for at least 10 seconds.
^ PCM in Closed Loop fuel control.
^ MAF between 2 and 35 grams per second
^ Actual idle speed less than desired idle speed by at least 80 RPM.
^ Vehicle speed between 30 and 45 MPH.
^ Commanded IAC position between 100 and 205 counts.
^ Throttle position varies less than 1 degree throughout the test.
^ Engine speed varies less than 75 RPM throughout the test.
^ A P/N to D/R or a D/R to P/N transition has not occurred within the last 60 seconds.

Conditions for Setting the DTC

MAF changes less than 1.5 grams per second during the 2 seconds that the IAC position was increased.

Action Taken When the DTC Sets

^ The PCM will illuminate the Malfunction Indicator Lamp (MIL) when the diagnostic runs and fails.
^ The PCM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame and Failure Records.

Conditions for Clearing the MIL/DTC

^ The PCM will turn the MIL OFF after three consecutive drive trips that the diagnostic runs and does not fail.
^ A Last Test Failed (current) DTC will clear when the diagnostic runs and does not fail.
^ A History DTC will clear after forty consecutive warm-up cycles with no failures of any emission related diagnostic test.
^ Use a scan tool to clear DTCs.
^ Interrupting PCM battery voltage may or may not clear DTCs. This practice is not recommended.

Test Description

Number(s) below refer to the step number(s) on the Diagnostic Table.

2. Checking if fault is present.
4. Checking for a circuit fault.
5. Checking for deposits, blockage, etc.