Spark plugs
Fire the fuel in each cylinder. Cheap, and changed every 30-100k miles depending on the plug.
Signs it's worn: Rough idle, misfires, poor mileage, hard starts.
What's wrong with my car? · Engine
Rough idle, shaking, or a flashing check-engine light?
Usually: Misfire: spark plugs or ignition coils.
Rough running, shaking at idle or a flashing check-engine light is usually a cylinder not firing: worn plugs, or a coil that's given up.
The parts that fix it
Ignition coils
Turn 12 volts into the high voltage the spark plugs need. Usually one per cylinder.
Signs it's worn: Check-engine light with a misfire code, shaking at idle.
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