Rough idling specifically when in park with a P0505 code in your 2017 Subaru Forester indicates the idle control system isn't maintaining stable engine speed. This code relates to the engine's ability to regulate idle RPM through the electronic throttle control system, which manages airflow for smooth idling.
Understanding P0505
P0505 signals an idle air control system malfunction. Modern vehicles like the 2017 Forester use electronic throttle control (drive-by-wire) where the ECM adjusts throttle position to maintain target idle speed. When this regulation fails or becomes erratic, the code triggers.
Why Rough Idle in Park
In park, the torque converter provides no load, making idle quality issues more apparent. Small RPM fluctuations felt as roughness in park may smooth out in drive where the transmission load dampens variations. This is why you notice the problem most when stationary.
Common Causes
Carbon buildup on the throttle body prevents precise air metering. Vacuum leaks introduce uncontrolled air. The throttle position sensor may send erratic signals. Mass airflow sensor inaccuracy affects idle fuel calculation. Spark plug or ignition issues create rough combustion at idle.
Electronic Throttle Factors
The Forester's electronic throttle body requires occasional cleaning and may need relearning after battery disconnection or repairs. Calibration drift or dirty throttle bore causes idle quality deterioration over time.