P1507

P1507 is a Subaru code meaning the Idle Air Control system cannot maintain the commanded idle speed — the ECU is asking the IAC stepper motor to open or close to hit ~700-750 rpm target idle and the actual rpm is drifting outside the allowable window (typically +/- 50 rpm for more than 10 seconds). On EJ-series Subarus the IAC is a stepper motor mounted to the throttle body that meters bypass air around the throttle plate; it operates from 0 to about 200 counts of step position, with idle typically sitting at 30-80 counts depending on engine temp, load, and accessory draw. Cheapest-first ladder: (1) Clean the IAC port BEFORE anything else. Pull the IAC motor (two screws on the throttle body), inspect the bypass passage and the pintle for carbon buildup, and clean both with throttle-body cleaner and a soft brass brush. EJ-series engines carbon-build the IAC bore at 80,000-120,000 miles to the point where the stepper can't open the bypass enough to maintain idle even at max counts. (2) Live-data scan: IAC count vs. actual rpm at warm idle. If counts are pegged at 180-200 (max-open) and rpm is still below target, the bypass is blocked OR there's a vacuum leak that the IAC can't compensate for; smoke-test the intake. (3) If counts are at 0-10 (max-closed) and idle is high, look for a vacuum leak the IAC is trying to compensate around or a sticking throttle plate. (4) Resistance-check the IAC stepper windings — EJ-series spec is typically 9-15 ohms across each pair, check WSM for your year. (5) Most expensive: replace the IAC motor ($120-$200 OEM). Clean the IAC port FIRST — EJ-series carbon-builds the IAC bore at 80K-120K mi and a stuck-closed IAC reads identical to a failed stepper.

Vehicle-specific patterns: 1999-2005 Subaru Legacy/Outback/Forester/Impreza EJ25 SOHC is the volume P1507 platform — the IAC bypass passage in the throttle body carbons up from blow-by routed through the PCV and from intake-valve carbon backing up at shutdown. Most cases clear with a 30-minute throttle-body and IAC cleaning. 2002-2007 WRX EJ205 turbo throws P1507 from the same IAC carbon issue compounded by turbo oil mist in the intake tract; clean both the IAC and the throttle body backside. 2004-2008 Forester XT and 2008-2014 Outback 2.5XT EJ255 see P1507 less often because the boosted engines run higher idle airflow that self-cleans the bypass, but when they do hit, it's usually a true IAC stepper failure rather than carbon. 2009-2013 Forester N/A with the EJ253 transition engine throws P1507 from a known IAC connector corrosion issue — pull the connector, inspect the pins for green oxide, and clean with electrical contact cleaner. Estimated repair: $0 (cleaning DIY) to $350 (IAC replacement at a shop with diagnostic time).