1. Preliminary Checks:
* Clean the IAC Valve: This is the most common cause. Dirt and grime can prevent the valve from operating smoothly. Locate the IAC valve (usually on the throttle body), disconnect it, and carefully clean it with a throttle body cleaner. Make sure you don't damage the delicate internal components. Reinstall it and see if the problem persists.
* Inspect the Vacuum Lines: Check all vacuum lines connected to the IAC valve and the throttle body for cracks, leaks, or disconnections. Replace any damaged lines.
* Check the Throttle Body: A dirty throttle body can also contribute to idle issues. Clean it thoroughly with throttle body cleaner, following the instructions on the cleaner. Be careful not to damage the throttle position sensor (TPS).
* Battery Voltage: Ensure your battery is properly charged and the charging system is working correctly. Low voltage can cause erratic idle and set this code.
2. More Advanced Troubleshooting (If cleaning doesn't work):
* IAC Valve Testing: You can test the IAC valve using a multimeter to check its resistance. The exact resistance values will vary depending on the specific IAC valve, so consult a repair manual for your 1999 Taurus to find the specifications. A faulty IAC valve will need to be replaced.
* Throttle Position Sensor (TPS) Testing: A malfunctioning TPS can also cause idle problems and trigger the P1504 code. Test the TPS with a multimeter following specifications in a repair manual. A bad TPS needs replacement.
* PCM (Powertrain Control Module) Issues: In rare cases, the PCM itself might be faulty. This is the least likely cause but should be considered if all other components have been checked and replaced. PCM diagnosis typically requires specialized tools and knowledge. Consider taking it to a professional mechanic if you suspect a PCM problem.
* Vacuum Leaks: A significant vacuum leak elsewhere in the engine system can disrupt the idle air control. Thoroughly inspect all vacuum lines and components for leaks. A smoke test may be necessary to locate hidden leaks.
3. Repair and Verification:
* Replacement Parts: If you've identified a faulty component (IAC valve, TPS, vacuum lines), replace it with a new part from a reputable auto parts store. Ensure you get the correct part number for your 1999 Taurus.
* Clear the Code: After making repairs, use an OBD-II scanner to clear the diagnostic trouble code P1504.
* Test Drive: Test drive the vehicle to ensure the problem is resolved. If the code returns, re-evaluate your troubleshooting steps.
Important Notes:
* Repair Manual: A repair manual specific to your 1999 Ford Taurus is highly recommended. It will provide detailed diagrams, specifications, and troubleshooting procedures.
* Safety: Always disconnect the negative battery terminal before performing any electrical work.
* Professional Help: If you're not comfortable working on your vehicle's electrical system, it's best to take it to a qualified mechanic.
By following these steps systematically, you should be able to diagnose and resolve the P1504 code on your 1999 Ford Taurus. Remember to always prioritize safety and consult a professional if needed.
