Bad Spark Plugs Symptoms and Easy Diagnosis Practical Mechanic Answers to Frequently Asked Questions About Spark Plugs Why does your car feel fine at high RPM but weak down low? Worn spark plugs can cause delayed combustion, vibration at idle, poor cold starts, and engine warning lights. Reading a spark plug is a great way to check your engine's health. Mechanics often read spark plugs to diagnose fuel mixture, ignition, and engine condition. Here's a clear guide for