P0505: OBD-II code P0505 flags a fault in the idle air control (IAC) system — the PCM can't hold a steady idle because the IAC valve or throttle body is misbehaving.

Symptoms

• Erratic idle — the engine hunts up and down, revs unusually high at start-up, or falls low enough to stall at stops.
• Difficulty starting, especially cold, and a rough idle that briefly improves when the throttle is blipped.
• Stalling on deceleration or when coming to a stop as the IAC fails to bypass enough air past the closed throttle plate.

Common causes

• A failed or stuck idle air control valve (IAC) — the classic cause on older cable-throttle engines.
• A carbon-clogged throttle body or IAC passage restricting the bypass air path around the throttle plate.
• A vacuum leak drawing unmetered air past the throttle, forcing the PCM to command excessive IAC compensation.

What to do?

On cable-throttle engines, remove and clean the IAC valve and the idle bypass passage in the throttle body. On drive-by-wire engines, perform a throttle body relearn with a scan tool after cleaning. Smoke-test the intake for vacuum leaks. Replace the IAC valve if cleaning doesn't restore a steady idle.