P1508

Circuit Description
The Engine Control Module (ECM) controls engine idle speed by adjusting the position of the Idle Air Control (IAC) motor pintle. The IAC is a bi-directional stepper motor driven by two coils. The ECM applies current to the IAC coils in steps (counts) to extend the IAC pintle into a passage in the throttle body to decrease air flow. The ECM reverses the current to retract the pintle, increasing air flow. This method allows highly accurate control of idle speed and quick response to changes in engine load. If the ECM detects a condition where too low of an idle speed is present and the ECM is unable to adjust idle speed by decreasing the IAC counts, Diagnostic Trouble Code (DTC) P1508 will set, indicating a problem with the idle control system.

Conditions for Setting the DTC
- No scan tool test is being run.
- DTC(s) P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0201, P0202, P0203, P0204, P0351, P0352, P0401, P0402, P0404, P0405, P0406, P0440, P0442, P0443, P0446, P0449, P0452, P0453, P0461, P0462, P0463, P0464, P0502, P1106, P1107, P1111, P1112, P1114, P1115, P1121, P1122, P1171, P1404, P1441 and P1627 are not set.
- Barometric pressure (BARO) is above 10.9 psi (75 kPa).
- ECT is above 122 °F (50 °C).
- Engine speed is more than 100 - 200 rpm lower than desired idle, based upon coolant temperature.
- The engine has been running for at least 125 seconds.
- Vehicle speed is greater than 1 mph (1.6 kph).
- Canister purge duty cycle is above 10%.
- Ignition voltage is between 9.5 and 16.7 volts.
- The throttle is closed.
- Engine speed is lower than desired idle.
- All of the above conditions are met for 10 seconds.

Action Taken When the DTC Set
- The Malfunction Indicator Lamp (MIL) will not illuminate.
- A history DTC is stored.

Conditions for Clearing the MIL/DTC
- A history DTC will clear after 40 consecutive warm-up cycles without a fault.
- DTC(s) can be cleared by using the scan tool.

Diagnostic Aids
Check for the following conditions:
- Poor connection at the ECM or IAC motor - Inspect harness connectors for backed-out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal-to-wire connection.
- Damaged harness - Inspect the wiring harness for damage.
- Restricted air intake system - Check for a possible collapsed air intake duct, restricted air filter element, or foreign objects blocking the air intake system.
- Throttle body - Check for objects blocking the IAC passage or throttle bore, excessive deposits in the IAC passage and on the IAC pintle, and excessive deposits in the throttle bore and on the pintle plate.
- Large vacuum leak - Check for a condition that causes a large vacuum leak, such as a disconnected brake booster hose.

Reviewing the Failure Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

Steps 1 - 4:

Steps 5 - 10:

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
1. The On-Board Diagnostic (OBD II) System Check prompts the technician to complete some basic checks and store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data taken when the malfunction occurred. The information is then stored on the scan tool for later reference.
8. The replacement ECM must be reprogrammed. Refer to the latest Techline procedure for ECM reprogramming.