Possible causes

– IACV-AAC valve harness is open or shorted
– IACV-AAC valve poor electrical connection
– Faulty IACV-AAC valve

Tech notes

There is a factory service bulletin for the following Lincoln Models:

2005-2007 Town Car
2006 Mark LT
Lincoln Factory Service Bulletin OBDII Code P0505

When is the code detected?

The P0505 code is set when the IACV-AAC valve does not operate properly.

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)
– Engine hard to start
– Rough or erratic idle speed

P0505 LINCOLN Description

This system automatically controls engine idle speed to a specified level. Idle speed is controlled through fine adjustment of the amount of air which by-passes the throttle valve via Idle Air Control Valve IACV) and Auxiliary Air Control AAC) Valve. The IACV-AAC valve changes the opening of the air by-pass passage to control the amount of auxiliary air. This valve is actuated by a step motor built into the valve, which moves the valve in the axial direction in steps corresponding to the Engine Control Module (ECM) output signals.
One step of IACV-AAC valve movement causes the respective opening of the air by-pass passage. (i.e. when the step advances, the opening is enlarged.) The opening of the valve is varied to allow for optimum control of the engine idling speed. The crankshaft position sensor (POS) detects the actual engine speed and sends a signal to the ECM. The ECM then controls the step position of the IACV-AAC valve so that engine speed coincides with the target value memorized in ECM. The target engine speed is the lowest speed at which the engine can operate steadily. The optimum value stored in the ECM is determined by takig into consideration various engine conditions, such as during warm up, deceleration, and engine load (air conditioner, power steering and cooling fan operation).

LINCOLN Technical Service Bulletins for P0505 code