P1506 Idle Air Control Over Speed Error.
I posted in this thread about it:
click here

From the Ford TSP (Technical Service Publication):

In reply to:

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Diagnostic Trouble Code (DTC) P1506 indicates Self-Test has detected an IAC overspeed error.

Possible causes:
-- IAC circuit short to GND.
-- IAC assembly stuck open.
-- Vacuum leak.
-- Damaged IAC assembly.
-- Damaged throttle body.
-- Damaged PCM.

* Key on, engine running.
* With the engine at idle, listen for vacuum leaks.
* Inspect the entire inlet air system from the Mass Air Flow (MAF) sensor to the intake manifold for damage or leaks such as:
* Cracked or punctured inlet air tube.
* Loose inlet air tube at the air cleaner housing or throttle body.
* IAC valve assembly or gasket seal.
* Intake manifold assembly or gasket seal.
* EGR valve gasket seal.
* Vacuum supply connectors and hose.
* PCV valve, connectors and hose.

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