P0506

P0506 sets when the PCM measures actual idle RPM at least 100 RPM below target with the IAC commanded fully open, indicating restricted idle air or excessive load. This is the most-searched IAC code by a wide margin, and the cause is almost always carbon buildup or a sticking IAC pintle rather than an electrical fault. Cheapest-first diagnosis: at warm idle, scan target RPM versus actual RPM and IAC steps, a P0506 vehicle typically shows IAC pegged near max (180+ steps on GM, 60+ percent duty on Ford) while actual RPM lags target by 100 to 300 RPM. Pull the IAC and look for a black varnish ring on the pintle and a sooty deposit in the throttle body bore; clean both with throttle body cleaner and a soft brush. Check for a stuck-closed PCV valve, a binding throttle cable, or a dragging A/C compressor that loads the engine at idle. Run an idle relearn after cleaning. The expensive misdiagnosis is replacing the torque converter or transmission when the actual fix is a $10 PCV valve and 20 minutes of throttle body cleaning.

Vehicle-specific patterns: 1999 to 2007 GM 4.8L/5.3L/6.0L Vortec engines (Silverado, Tahoe, Suburban) set P0506 from cold-stall vacuum leaks at the upper intake manifold gaskets, GM TSB 06-06-04-037, the plastic manifold warps and the gasket lets in unmetered air at startup. 2003 to 2010 Chrysler 5.7L Hemi sets P0506 from a cracked PCV hose and a sticking IAC, plus intake manifold runner control issues. 2000 to 2006 Honda Accord 2.3L sets P0506 from carbon at the IAC and a sticking EGR valve. 1998 to 2004 Subaru Outback with EJ25 sets P0506 from intake manifold gasket leaks at the runners. Estimated repair: $40 to $650.