P1507
Idle Control System Malfunction (RPM Lower Than Expected)
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The P1507 code on Subaru vehicles indicates the idle control system is malfunctioning, specifically that the engine RPM is falling below the ECM's expected idle speed target. The idle air control valve (IACV) is responsible for maintaining a stable idle by regulating airflow that bypasses the closed throttle plate. When the IACV can't provide enough air, idle RPM drops too low and the engine may stall, especially when additional electrical loads like the A/C or headlights are switched on.
The most common and cost-effective fix is simply cleaning the IACV and throttle body. Over time, carbon deposits from PCV system vapors and oil mist accumulate on the IACV's internal valve, preventing it from opening fully. Removing the IACV, spraying it thoroughly with throttle body cleaner, and letting it dry often restores normal operation. While you have the IACV off, clean the throttle body too — this one-two punch of cleaning resolves P1507 in the majority of cases.
For manual-transmission Subarus, also consider the neutral safety switch, which is a $30 part that screws into the transmission. A failed switch can cause idle control issues. If cleaning the IACV and throttle body doesn't fix the code, check for vacuum leaks — a smoke test is the most effective method. This code should be addressed soon because unexpected stalling at intersections or in traffic is a safety concern.
Severity
Symptoms
- •Engine idle is noticeably too low and feels like it might stall
- •Engine stalls when coming to a stop or shifting to neutral
- •Rough or shaky idle that vibrates through the steering wheel
- •Engine may die when turning on A/C, headlights, or other electrical loads
- •Intermittent stalling when the engine is cold
Likely Causes
Carbon and oil deposits accumulate on the IACV's rotating valve over time, preventing it from opening fully. This restricts the bypass air that maintains idle speed, causing RPM to drop below the ECM's target.
Carbon buildup around the throttle plate and bore reduces the baseline airflow at idle. Combined with any IACV restriction, this pushes idle RPM below the expected range.
A vacuum leak from a cracked hose, loose intake manifold gasket, or failed PCV valve introduces unmetered air that disrupts the air/fuel ratio and causes unstable, low idle RPM.
On manual-transmission Subarus, a failed clutch or neutral safety switch can send an incorrect signal to the ECM, causing it to command an improper idle speed that triggers P1507.
The IACV motor itself may have failed electrically, preventing the valve from responding to ECM commands to increase idle airflow.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Remove and clean the idle air control valve (IACV) with throttle body cleaner
- Clean the throttle body to remove carbon deposits
- Check for and repair vacuum leaks in hoses and intake manifold gaskets
- Replace the neutral safety switch (manual transmission models)
- Replace the IACV if cleaning does not restore function