The code "P1504" refers to a malfunction in the Idle Air Control (IAC) system. The IAC system is responsible for controlling the engine idle speed, ensuring that it stays within a specific range while the car is not in motion.

When the P1504 code is triggered, it means that there is a problem with the IAC system. This could be due to a variety of reasons, such as a faulty IAC valve, a vacuum leak, or a problem with the wiring connected to the IAC system.

To diagnose and fix the issue, you can start by visually inspecting the IAC valve and its connections for any signs of damage or debris. Clean or replace the valve if necessary. Next, check for any vacuum leaks by inspecting all hoses and connections in the engine bay. Leaks can often be fixed by replacing or repairing the affected components.

If the issue persists, you may need to use a diagnostic tool to read the live data from the IAC system and perform further testing. This will help identify any electrical or sensor-related problems that may be causing the malfunction. Depending on the results, additional repairs or component replacements may be necessary.

Remember, if you are not experienced with car mechanics, it's always best to consult a professional technician who has the knowledge and proper tools to diagnose and fix the issue correctly.