On the 2000 Ford F-150, the P1504 is pointing to a problem in the Idle Air Control System. The IAC Valve is located on the top of the engine, mounted to the side of the throttle body. With the key on / engine off, take a voltage reading at the electrical connector. The voltage should be 10.5v or greater. If not, there is a problem in the circuit. Inspect the wire harness for any loose, broken of damaged wires.
If the voltage checks ok, take an Ohms reading across the pins on the IAC Valve itself. The Ohms reading should be between 6 and 13 ohms. If not, the IAC Valve will need to be replaced.
If these all check ok, make sure there is no binding of the throttle plate inside the throttle body (remove the air duct to gain access). Inspect for any excessive build-up. If found, throttle cleaner and a toothbrush work well here. Make sure to thoroughly clean the bore, plate and plate pivot points. Inspect the air filter itself and make sure it is not excessively dirty. Make sure there are no rips or holes in the air filter housing to throttle body duct. If these all check ok, run an intake air leak test: With the engine running, carefully and sparingly spray some throttle cleaner around the base of the intake. If the spray makes the engine respond, this is a sign of a leak and the intake gasket will need to be replaced (make sure to visual inspect the manifold itself for any cracks at this time as well).
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