This tool provides general guidance — not professional mechanical advice. Always have a qualified mechanic inspect your vehicle before making any repair decisions.
#1 Most Likely
Engine shaking or rough at idle
Typical Cost
$50 – $600
Spark plug replacement typically costs $50–$200 per industry repair data (2024) depending on vehicle and plug type. Ignition coil replacement may cost $213–$383. Vacuum leak repair varies widely. Urgency reflects worst-case cause (catalytic converter damage from persistent misfire). Prices vary by vehicle make and region.
Wide range because the exact repair depends on what's causing the symptom.
- Worn or fouled spark plugs
- Failing ignition coil causing cylinder misfire
- Vacuum leak affecting fuel-air mixture
- Clogged or failing fuel injector
- Low engine compression from worn rings or valves
An engine that shakes or runs roughly while idling may indicate worn spark plugs, a misfiring cylinder, a vacuum leak, or a failing fuel injector. In the worst case, a persistent misfire can damage the catalytic converter and lead to more extensive repairs if left unaddressed.
What a mechanic will check
A mechanic will likely retrieve engine fault codes, check spark plug condition, inspect ignition coils, and inspect vacuum hoses for leaks. Fuel injector balance testing may be performed if the misfire is fuel-related.
What to say to your mechanic
“My engine shakes and runs roughly when I am stopped or idling. Can you check my spark plugs, ignition coils, and pull any fault codes to tell me what may be causing the rough idle? I want to understand how urgent this is.”
Script based on industry repair procedures