P0505

P0505 is the generic Idle Air Control system fault and applies only to cable-throttle vehicles, not drive-by-wire (ETC) cars. The PCM commands the IAC valve, a stepper motor or pintle that meters air around a closed throttle plate, and watches actual RPM versus target. If commanded position and resulting RPM disagree by more than the allowable window, P0505 sets. Cheapest-first diagnosis: scan IAC step count or duty cycle at warm idle in park with all accessories off, a healthy IAC typically sits between 15 and 40 steps (GM) or 25 to 45 percent duty (Ford). If the count is pegged at 0 or at the max (commonly 180 to 255 steps), the PCM has run out of authority, which means either a vacuum leak (high steps) or a stuck-open IAC/leaking gasket (low steps). Smoke-test the intake for leaks before touching the IAC, this finds the fault in roughly half of P0505 cases. Remove the IAC and clean the pintle and seat with throttle body cleaner, then perform an idle relearn (key on 5 minutes, key off 30 seconds, restart and idle 3 minutes with no load). The expensive misdiagnosis is buying a new IAC and a new throttle body when a $15 PCV valve and a smoke test would have solved it.

Vehicle-specific patterns: 1996 to 2005 GM 4.3L/4.8L/5.3L/6.0L trucks set P0505 from a clogged IAC pintle, GM TSB 03-06-04-030 covers the cleaning procedure. 1996 to 2000 Honda Civic and CR-V set P0505 from a failing IAC valve solenoid winding, Honda part 16022-P2A-J01. 1998 to 2003 Ford Ranger and Explorer with 4.0L SOHC set P0505 from carbon at the IAC bore, plus a leaking lower intake gasket. 2000 to 2004 Jeep Wrangler TJ with 4.0L sets P0505 from an aged IAC stepper motor. Estimated repair: $25 to $280.