28/08/2026
YOU KEEP REPLACING SPARK PLUGS FOR A PROBLEM THE PLUGS NEVER CAUSED.
MYTH: "Rough idle, misfire, Check Engine Light — it's always the spark plugs."
TRUTH: Plugs get blamed because they're cheap and easy to swap. But sitting directly on top of every spark plug in a modern engine is an ignition coil — and a failing coil produces the exact same symptoms as a bad plug. Replace the plug, leave the coil, and the "new" plug will misfire again within weeks. It was never the plug's fault.
Here's what a dying coil actually looks like, stage by stage:
🔹 STAGE 1 — THE WHISPER A slightly rough idle on cold starts, or only when it's raining, that smooths out once the engine warms up. Nobody notices this stage — it feels like "the car just needs a minute."
🔹 STAGE 2 — THE FLICKER Intermittent misfire under acceleration or climbing a hill — a small stumble you can feel but can't quite explain. The Check Engine Light flickers on, then off, then on again. This is the coil managing to fire at idle but falling short the moment the engine demands a stronger spark.
🔹 STAGE 3 — THE PATTERN The misfire becomes consistent, not occasional. Rough idle all the time, not just on cold starts. The Check Engine Light stays on — or starts flashing, which means unburned fuel is now hitting your catalytic converter every time that cylinder misfires.
🔹 STAGE 4 — DEAD CYLINDER That cylinder stops firing altogether. Noticeable power loss, a rougher engine note, and — if it's been flashing a while — real risk of catalytic converter damage that turns a coil job into a much bigger bill.
THE PART NOBODY TELLS YOU: An ignition coil is a mini transformer — it takes your battery's 12 volts and steps it up to 20,000–40,000+ volts to jump the spark plug gap. Sitting right on top of the plug means it lives in constant heat and vibration, and over time that breaks down the insulation inside its windings. A coil with degrading insulation can usually still deliver enough voltage at idle, when demand is low — but under load, when the engine needs a stronger, hotter spark to fire against higher cylinder pressure, it comes up short. That's exactly why the classic tell is "runs fine at idle, misfires under acceleration." It's not a random fault. It's a coil that's already failing, just not failing hard enough yet to show up standing still.
THE FREE DIAGNOSTIC TEST: If a code points to a specific cylinder, swap that coil with the one on another cylinder. If the misfire code follows the coil to its new position, it's the coil. If it stays on the original cylinder, look elsewhere. Five minutes, zero cost, and it settles the argument before you spend money guessing.
THE BRANDS, HONESTLY:
NGK and Denso — OE-grade on most Japanese platforms, reliable winding insulation
Bosch — OE-grade, common across European and some Asian platforms
Cheap aftermarket coils exist and some are fine, but thin, low-grade insulation is exactly where budget units fail fastest — especially in heat
Fix the coil, not just the plug it's sitting on. Otherwise you're paying twice for the same misfire.