According to the Ford TSP (Technical Service Publication), the first thing you should do is check for vacuum leaks:
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.
96 Contour SE
Duratec V6 24-valve 2.5L ATX
108,000 mi.
Replaced: crankshaft w/ DMD, EGR valve (clogged), EVR, DPFE, PCV valve, evap emissions hose & tube (cracked), window regulator (broken), LH & RH PCV tubes, UIM gasket (leak), ignition coil
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