Engine Shaking / Misfiring

What's happening

Most likely causes

1. 1

   Worn spark plugs

   The top cause. Plugs wear out, the gap widens, and the spark gets weak under load.

2. 2

   Failing ignition coil

   One weak coil kills one cylinder. Swap coils between cylinders to confirm.

3. 3

   Clogged fuel injector

   Starves one cylinder of fuel; often worse when cold.

4. 4

   Vacuum leak

   Lean mixture causes random misfires, usually with a lean code too.

5. 5

   Low compression

   Worn rings or valves on high-mileage engines. A compression test tells the truth.

How to diagnose it — in order

1. 1

   Scan for misfire codes: P0300 is random, P0301-P0312 name the exact cylinder.

2. 2

   Start with plugs; they are cheap and the most likely fix.

3. 3

   If one cylinder is named, swap its coil to another cylinder and see if the code follows.

4. 4

   Check for vacuum leaks with a smoke test or carb cleaner around intake gaskets.

Trouble codes tied to this symptom

If your scanner shows one of these, read its dedicated trouble-code page for the full diagnosis.