The Acura RLX may have issues with the idle air control system, causing:
The idle air bypass system is made up of vacuum lines, an idle air control valve (IACV), the throttle body, and intake manifold, and allows enough air into the engine to idle when the throttle body is closed. The OBD trouble code P0505 refers you to this system to inspect for failures.
The most likely cause is a dirty or failed IACV, but vacuum lines, intake manifold gasket, throttle body gasket, and IACV gasket should be inspected.
In all cases involving the IACV, the throttle body ports should be cleaned prior to installing the IACV onto the throttle body.