P0508

P0508 indicates the IAC control circuit is reading low voltage, typically a short-to-ground on the IAC driver wire or an internally shorted IAC solenoid winding. The PCM detects that the commanded current exceeds the expected return and sets the code, often with the IAC disabled and idle dropping to a stall or surging. Cheapest-first diagnosis: key on engine off, disconnect the IAC and measure resistance across the solenoid terminals, a healthy 2-coil IAC reads roughly 8 to 14 ohms per coil (GM 4-pin) or 6 to 13 ohms single-coil (Ford 2-pin); anything under 4 ohms is a shorted winding and the IAC is the fault. With the IAC disconnected and the engine running, back-probe each driver wire at the PCM connector and read voltage, you should see roughly 7 to 10V key-on engine running on the driven pin. A flat 0V on a driver wire with the IAC unplugged means the wire is shorted to ground between the connector and the PCM. Check for chafed harness at the valve cover or at the intake manifold mounting bolts. The expensive misdiagnosis is replacing the PCM when a $25 IAC valve or a section of repair tape on the harness fixes it.

Vehicle-specific patterns: 1996 to 2002 GM 4.3L/5.0L/5.7L Vortec sets P0508 from an internally shorted IAC winding, the failure rate rises sharply past 100k miles. 1997 to 2003 Ford F-150 4.6L and 5.4L set P0508 from a chafed IAC harness at the upper intake bolt where the wire rubs through. 2000 to 2005 Chrysler 3.3L/3.8L minivans set P0508 from a shorted IAC pigtail and water intrusion at the connector. 1998 to 2002 Honda Accord 3.0L V6 sets P0508 from a failing IAC valve at the throttle body. Estimated repair: $35 to $220.