Fault code P1504 – a step-by-step guide to diagnosing and fixing it

Error code P1504 is displayed and the check engine light is on - what does this mean?

The OBD/EOBD system detects a fault in the idle air control valve or the idle control. The DTC P1504 indicates a problem with the air supply at idle, which can lead to unstable idle speeds. This manufacturer-specific error code occurs particularly frequently in vehicles from the VAG Group (Volkswagen, Audi, Seat, Skoda).

What can cause this error code?

• Defective idle air control valve (IAC valve)
• Dirty or blocked idle air control valve
• Damaged vacuum hoses for idle control
• Faulty wiring to the idle control motor
• Defective throttle valve or throttle valve sensor
• Leaks in the intake system
• Dirty mass air flow sensor
• Defective engine control unit
• Loose or corroded plug connections

What symptoms occur in parallel with error code P1504?

• Unsteady idling: The engine runs unevenly at idle or fluctuates in speed
• Engine stalling: The vehicle stalls when stopping or idling
• Starting problems: Difficult starting, especially when the engine is cold
• Increased fuel consumption: Unstable combustion increases fuel consumption
• Jerky ride: Uneven engine running at low engine speeds
• Poor throttle response: Delayed response when accelerating from idle speed

How can the problem with error code P1504 be rectified?

• The repair requires a systematic diagnosis of the idle control components.
• Remove the idle air control valve, clean thoroughly or replace completely if defective
• Check all vacuum hoses for cracks, leaks or loose connections and replace defective hoses
• Dismantle the throttle valve, clean with a special cleaner and have the throttle valve sensor calibrated
• Check wiring to idle speed actuator for damage, corrosion or short circuits and repair
• Remove the mass air flow sensor, clean or replace with a new sensor if irreparably damaged
• Check the entire intake system for leaks and replace defective seals or hoses
• Read out the engine control unit, delete the fault memory and replace the control unit if it occurs repeatedly