Possible causes

– Electronic Throttle Control System (ETCS).
– Air induction system
– PCV hose connection

Tech notes

Before replacing any parts, try adjust engine idle first.

When is the code detected?

The ECM regulates the idle speed by opening and closing the throttle valve using the ETCS. The ECM concludes that the idle speed control ECM function is malfunctioning if: 1) the actual idle RPM varies more than the specified amount five times or more during a drive cycle, or 2) a learning value of the idle speed control remains at the maximum or minimum five times or more during a drive cycle. The ECM will turn on the MIL and set a DTC.
Example: If the actual idle RPM varies from the target idle RPM by more than 200* rpm five times during a drive cycle, the ECM will turn on the MIL and will set a DTC.

Symptoms

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

P0505 LEXUS Description

The idle speed is controlled by the Electronic Throttle Control System (ETCS).The ETCS is composed of the throttle motor, which operates the throttle valve, and the throttle position sensor, which detects the opening angle of the throttle valve.The Engine Control Module (ECM) controls the throttle motor to provide the proper throttle valve opening angle to obtain the target idle speed.