P1508

Steps 1 - 3:

Steps 4 - 8:

DTC P1508 IAC System Low RPM




Circuit Description
The powertrain control module (PCM) 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 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 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 detects a condition where too low of an idle speed is present and the PCM is unable to adjust idle speed by increasing 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: TP sensor, VSS, ECT, EGR, fuel system, MAF, MAP, IAT, canister purge, injector control or ignition control.
- Barometric pressure is above 75 kPa.
- Engine coolant temperature (ECT) is above 50 °C (120 °F).
- Vehicle speed is less than 1 mph.
- The engine has been running for at least 125 seconds.
- Canister purge duty cycle is above 10%.
- Ignition voltage is between 9.5 volts and 16.7 volts.
- The throttle is closed.
- Engine speed is lower than desired idle.
- Engine speed is more than 100-200 RPM lower than desired idle, based upon coolant temperature. All of the above conditions are met for 5 seconds.

Action Taken When the DTC Sets
- The PCM will illuminate the malfunction indicator lamp (MIL) after the second consecutive trip in which the fault is detected.
- The PCM 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
- The PCM will turn the MIL OFF on the third consecutive trip cycle during which the diagnostic has been run and the fault condition is no longer present.
- 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 PCM battery feed.

Diagnostic Aids
Check for the following conditions:
- Poor connection at PCM 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 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 throttle plate.
- Large vacuum leak - Check for a condition that causes a large vacuum leak, such as an incorrectly installed or faulty PCV valve or 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.