P1508
DTC P1508 IDLE AIR CONTROL COUNTS TOO LOW
Circuit Description
The powertrain control module (PCM)/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 PCM/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 PCM/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 PCM/ECM detects a condition where too low of an idle speed is present and the PCM/ECM is unable to adjust idle speed by decreasing the IAC counts, DTC P1508 will set, indicating a problem with the idle control system.
Conditions for Setting the DTC
- No scan tool test is being run.
- None of these DTCs are set: Throttle Position (TP) sensor, Vehicle Speed Sensor (VSS), Engine Coolant Temperature (ECT), Exhaust Gas Recirculation (EGR), Fuel System, Mass Airflow, Manifold Absolute Pressure (MAP), Intake Air Temperature (IAT), Canister Purge, Injector Control or Ignition Control.
- Barometric pressure (BARO) is above 75 kPa.
- Engine Coolant Temperature (ECT) is above 50 °C (120 °F).
- 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.
- 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 PCM/ECM will not illuminate the Malfunction Indicator Lamp (MIL).
- The PCM/ECM will store conditions which were present when the DTC was set as Freeze Frame and in the Failure Records data.
Conditions for Clearing the MIL/DTC
- A history DTC P1508 will clear after 40 consecutive warm-up cycles have occurred without a fault.
- DTC P1508 can be cleared by using the scan tool CLEAR INFO function or by disconnecting the battery.
Diagnostic Aids
Check for the following conditions:
- Poor connection at the PCM/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 To 4:
Steps 5 To 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 PCM/ECM must be reprogrammed. Refer to Powertrain Control Module / Service and Repair.