You might have a bad or stuck purge valve in the EVAP system. That will generate a P1506 according to the Ford manual.
KE20 DTC P1506: 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 valve stuck open.
Vacuum leak.
Damaged IAC valve.
Damaged throttle body.
Damaged PCM.
Key on, engine running.
With the engine at idle, listen for vacuum leaks.
Inspect the entire intake air system from the mass air flow (MAF) sensor to the intake manifold for damage or leaks such as:
Cracked or punctured intake air tube.
Loose intake air tube at the air cleaner housing or throttle body.
IAC valve or gasket seal.
Intake manifold assembly or gasket seal.
EGR valve gasket seal.
Vacuum supply connectors and hose.
PCV valve, connectors and hose.
Key off.
Were any leaks detected in the above areas?
Yes
REPAIR as necessary. RESTORE vehicle.
No
COMPLETE PCM Reset to clear DTCs. RERUN Quick Test . GO to KE21 .
KE21 CHECK EVAP SYSTEM FOR A STUCK OPEN VALVE
Disconnect hoses at EVAP canister purge valve (or VMV).
Connect a hand vacuum pump at the fuel vapor port to EVAP canister at the EVAP canister purge valve (or VMV).
Apply 53 kPa (16 in-Hg) of vacuum to EVAP canister purge valve (or VMV).
Does the EVAP canister purge valve (or VMV) hold vacuum for 20 seconcds?
Yes
RECONNECT hoses. GO to KE22 .
No
REPLACE EVAP canister purge valve. RESTORE vehicle. COMPLETE PCM Reset to clear DTCs. RERUN Quick Test .
