Why does my car shake at idle?

Vibration that's worst at idle and smooths out as RPMs rise is classic misfire territory at low RPM, where the firing pulses are far apart and any one weak cylinder is obvious. Combine that with a check engine light (or even just a flashing one) and you've nearly confirmed it. If the engine itself rocks side-to-side instead of vibrating, look at broken motor mounts — a quick visual under the hood while a helper revs it gives you the answer.

Get a free AI-ranked diagnosis for your exact year, make and model — with fair repair cost ranges and DIY-vs-shop guidance.