Summary
P0506 means the engine is idling lower than the PCM's target — typically 100+ RPM below the expected idle speed. The most common causes are a dirty or stuck IAC valve (35%), a vacuum leak causing unstable idle (25%), and a dirty throttle body (20%). A low idle can lead to stalling at stops. Start by cleaning the IAC valve and throttle body ($5–$10 in cleaner) — this fixes most P0506 cases without replacing parts.
Severity: Moderate
Safe to drive: Yes, but risk of stalling at stops
Repair cost: $5–$300 depending on cause
DIY difficulty: Easy to Moderate
What does P0506 mean?
The PCM controls idle speed by adjusting the amount of air bypassing the throttle plate — through an idle air control (IAC) valve on older vehicles or by adjusting the electronic throttle body directly on newer ones. The warm idle target is typically 600–800 RPM in park/neutral with no electrical loads.
P0506 sets when the actual idle speed falls below the PCM's target by a sustained margin (usually 100–200 RPM) and the PCM has exhausted its ability to raise it. The PCM commands more air through the IAC or opens the throttle slightly, but the RPM still won't come up to the target.
This is the opposite of P0507 (idle too high). You'll notice the engine lugging or stumbling at idle, the tachometer sitting below its normal position, and the engine may stall when you come to a stop or shift into drive.