Signs Bad Spark Plugs – What They Look Like and Why It Matters

When working with bad spark plugs, spark plugs that are worn, fouled or incorrectly gapped, causing poor ignition. Also known as faulty spark plugs, they can trigger a range of engine problems that most drivers overlook. Recognising the warning signs early can save you from costly repairs and keep your car running smooth.

One of the quickest clues is a misfire, a sudden loss of power when a cylinder fails to fire. A misfire often shows up as a rough idle, stumbling on acceleration, or a noticeable shake at low speeds. Bad spark plugs are a common cause because they don’t deliver the spark needed to ignite the air‑fuel mix.

A failing plug also drags down fuel economy, the distance a car travels per litre of fuel. When the spark is weak, the engine compensates by using more fuel, which means you’ll see the MPG drop and the fuel gauge jump faster. Over time, the extra unburnt fuel can poison the catalytic converter, the emissions device that turns harmful gases into harmless ones. A clogged converter can trigger a check‑engine light and lead to expensive replacement.

The ignition system hinges on the plug’s ability to create a high‑voltage spark at the right moment. If the gap is off, the voltage spikes, and the plug may wear out faster. A simple test – removing the plug and checking the electrode wear or using a plug tester – can tell you if it’s time for a swap. Typical UK replacement costs range from £15 for a basic plug to £80 for a premium iridium type, and most jobs take under an hour.

Besides misfires, low MPG, and a check‑engine warning, you’ll notice hard starts, a popping sound from the exhaust, or a noticeably rough ride at idle. All these clues point back to the same root: the spark plug isn’t doing its job.

What to Expect from Our Guides

Our collection covers everything from quick visual inspections to detailed testing procedures, so you can decide whether a DIY fix or a professional service is right for you. Dive into the posts and arm yourself with the knowledge to keep your engine firing perfectly.