Summary
P0505 means the PCM has detected a general fault in the idle speed control system — the engine isn't idling at the commanded speed or the idle control mechanism isn't responding correctly. The most common causes are a dirty or stuck idle air control (IAC) valve (40%), a vacuum leak (25%), and a dirty throttle body (15%). Start with a throttle body and IAC cleaning ($5–$10 in cleaner) — this resolves most P0505 cases without replacing parts.
Severity: Moderate
Safe to drive: Yes, with caution — erratic idle may cause stalling
Repair cost: $5–$300 depending on cause
DIY difficulty: Easy to Moderate
What does P0505 mean?
The PCM controls engine idle speed by regulating airflow through either an idle air control (IAC) valve (on cable-throttle vehicles) or the electronic throttle body directly. The target idle speed is typically 600–800 RPM with the engine warm, in park or neutral, and no electrical loads.
P0505 is a general code indicating the idle control system isn't functioning as expected. Unlike P0506 (idle too low) or P0507 (idle too high), P0505 can mean the idle speed is erratic, unresponsive to PCM commands, or the IAC valve isn't moving when commanded. Some manufacturers set P0505 when the PCM can't achieve stable idle within its correction range.
You may notice the engine idling too high, too low, hunting up and down, or stalling at stops. The symptoms can vary depending on whether the IAC is stuck open, stuck closed, or intermittently responsive.