Engine shaking, stumbling, or rough at idle? Something is misfiring — the question is what.

What it usually proves to be (in order)

1. 1A dead coil or plug on ONE cylinder (P0301) — cheap, provable in minutes
2. 2A clogged or leaking injector (needs a balance test, not a parts cannon)
3. 3Random misfire across cylinders (P0300) — fuel delivery, vacuum leak, or timing
4. 4Low compression or a mechanical problem — rare, expensive, and the last thing shops test

What a parts-changer will tell you

Plugs and coils — all of them ($500–$900). A reasonable-sounding shotgun that misses injector, fuel, and mechanical causes entirely — and cooks your catalytic converter if the real problem stays.

Typical guess-and-swap cost: $500–$900 (ignition parts cannon) when fuel or timing is at fault

The test that proves it

Per-cylinder misfire counters, swap-the-coil logic, injector balance, compression and leak-down if the pattern says mechanical. Random vs. specific-cylinder misfires are completely different diagnoses.