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P1504

Intake Air Control Circuit Malfunction.

  • Lincoln
  • Powertrain
  • Manufacturer code
  • Medium severity

How this code is built

P
SystemPowertrain
1
Defined byThe manufacturer
5
AreaVehicle speed, idle control and auxiliary inputs
04
FaultThe specific fault in that area

What it means

On Lincoln models P1504 means the powertrain control module has detected a fault in the intake air control circuit, which on Ford-group vehicles is the idle air control (IAC) valve circuit that regulates idle speed by letting air bypass the throttle plate. The module sets the code when the circuit's electrical feedback or the actual idle speed does not match what it commanded. The exact layout depends on the model and engine, as some use a separate IAC valve and others handle idle through the throttle body.

Symptoms

  • Rough, hunting or surging idle
  • Engine stalls at idle or when slowing to a stop
  • Idle speed too high or too low
  • Hard starting, especially when cold
  • Malfunction indicator lamp on
  • Engine stumbles when the air conditioning or steering load comes on

Possible causes

  • Dirty or carbon-clogged idle air control valve or bypass passagelikely
  • Faulty idle air control valve (internal open or short circuit in the solenoid or motor)likely
  • Damaged wiring, chafed loom or corroded connector at the IAC valvelikely
  • Vacuum leak or unmetered air entering after the throttle body, upsetting idle controlpossible
  • Poor power or earth supply to the valve or control modulepossible
  • Fault within the powertrain control module driver circuitrare

How to diagnose

  1. Read the stored codes and freeze frame data to see engine temperature, speed and load when the fault set, and note any other codes, especially lean or idle-related ones
  2. Check the live data for commanded versus actual idle speed and the IAC duty cycle or position, if the model supports it
  3. Visually inspect the IAC valve wiring and connector for chafing, heat damage, corrosion and bent or pushed-back pins
  4. Inspect the intake hoses, throttle body and manifold joints for splits, loose clips and vacuum leaks
  5. Remove the IAC valve and inspect it and its air passage for carbon build-up, and check the valve moves freely
  6. Measure the valve coil resistance and compare it with the specification for that model and engine
  7. With the connector disturbed, check for power, earth and the control signal from the module using a multimeter, following the wiring diagram for the vehicle
  8. Wiggle-test the harness with the engine idling to find intermittent faults

How to fix

  • Clean the IAC valve and bypass passage using a cleaner suitable for throttle and intake components, then refit and complete any idle relearn procedure
  • Replace the IAC valve if its resistance is out of specification or it sticks or fails to respond
  • Repair or replace damaged wiring and corroded or loose connector terminals
  • Repair any vacuum leaks by replacing split hoses, gaskets or seals
  • Restore faulty power or earth connections found during testing
  • Replace or repair the powertrain control module only after all wiring and the valve have been proven good

Can I keep driving?

Medium severity

The car can usually be driven short distances, but stalling at junctions or low speeds can be unsafe, so get it fixed soon. Avoid driving if it stalls repeatedly.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.

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