When you're sitting at a stop light and the tachometer keeps swinging - up to 1,500, down to 600, back up - the engine is hunting for a steady idle and can't hold it. This is almost always one of three things: a vacuum leak letting in unmetered air, a dirty throttle body, or a failing idle air control valve. All three are fixable for under $100 in parts on most cars.
Hunting idle by itself isn't dangerous - the engine isn't damaging anything. But if the swings get bad enough that the car stalls at red lights, you've got a safety problem. Diagnose within a week or two so it doesn't leave you stalled in traffic.
The classic vacuum leak code. The computer is trying to idle at 700 RPM but unmetered air is sneaking past, pushing it higher. Usually a cracked vacuum hose or leaking intake gasket. Often $5-30 to fix.
View Full Diagnosis - P0507 →The opposite of P0507 - the engine is dragging below normal idle speed. Usually a dirty throttle body or a failing idle air control valve. A throttle body cleaning ($7 for the spray) fixes this surprisingly often.
View Full Diagnosis - P0506 →The IAC valve controls airflow at idle. When it gets stuck or dirty, idle gets erratic. On older cars (pre-2010ish) the IAC is a separate valve you can clean. On newer cars it's integrated into the throttle body.
View Full Diagnosis - P0505 →A vacuum leak that throws P0507 will often also throw P0171. The computer adds fuel to compensate for the extra air, but it can't fully catch up - so idle wanders.
View Full Diagnosis - P0171 →Describe your symptom (or paste your code) and our AI gives you the exact most-likely fix, parts list, and cost - in under 30 seconds. $5.99. One report, no subscription.
Get My Repair Report →Saves the $150 shop diagnostic fee.
If your scan tool is showing one of these codes alongside this symptom, that's your starting point. Click any code for the full diagnosis, common causes, and repair costs.
Three main causes: a vacuum leak (unmetered air sneaking into the intake), a dirty throttle body (carbon buildup disrupting smooth airflow), or a failing idle air control valve. All three confuse the computer about how much air is entering, so it can't hold a steady idle.
Not dangerous to the engine - it's not damaging anything by itself. But if the idle drops low enough to stall, you have a safety problem at intersections. Get it diagnosed within a couple of weeks.
Yes - this is one of the most common causes. Carbon and gunk build up on the throttle plate over time and disrupt the precise airflow needed at idle. Cleaning it with a $7 can of throttle body cleaner fixes this surprisingly often.
Cheapest fixes start at $5-15 (a single cracked vacuum hose). Throttle body cleaning is about $7 plus your time. An idle air control valve is typically $40-150 in parts. A new throttle body assembly runs $100-400. Pull your code first.