Spark Plugs: When to Change Them and What Happens If You Wait Too Long

The engine stumbles a bit when you start it in cold weather. Or you notice the car pulling a little less, consuming a little more fuel. Nothing dramatic. Nothing that would make you pull over to the shoulder. But something isn’t quite right.

In many cases, the culprit is a small, inexpensive part that has been working for years without anyone paying it much attention: the spark plug.

It’s not a spectacular breakdown. It doesn’t make noise or leave stains on the ground. It simply wears down silently, until the engine starts to feel its absence.

What a Spark Plug Does and Why It Matters

The spark plug creates the spark that ignites the mixture of fuel and air inside the cylinder. Without that spark, there’s no combustion. Without combustion, there’s no movement.

When it works well, the process happens thousands of times per minute with millimeter precision. When the spark plug wears out, the spark becomes irregular, weak, or fails outright in some cycles. The engine compensates as best it can, but at a cost: more fuel consumption, less power, and higher emissions.

A worn spark plug won’t destroy the engine immediately, but it can compromise its performance and, over time, damage the catalytic converter if unburned fuel reaches the exhaust system.

Signs That It’s Time to Change Them

There’s not always a warning light on. Sometimes the symptoms are more subtle:

• Hard starting, especially in cold weather or when humidity is high
• Jerking or hesitation when accelerating, especially during gentle acceleration from low speeds
• Increased fuel consumption without any change in driving style
• Rough idle, the engine vibrates or “misfires” when standing still
• Check engine light on constantly

None of these symptoms point directly to spark plugs. A scanner diagnostic at a shop will tell you more precisely whether the problem is there or in another system.

When to Change Spark Plugs Based on Type

This is where many drivers get confused, because not all spark plugs last the same amount of time. The interval depends on the electrode material:

Conventional spark plugs (nickel): these are the most common and the cheapest. Their lifespan is around 20,000 to 40,000 km, though it varies depending on the manufacturer and driving conditions.

Platinum or iridium spark plugs: these are more expensive, but they last much longer. According to manufacturers, they can reach between 80,000 and 100,000 km. Many modern cars come equipped with this type as standard. Check your vehicle’s manual to find out what type yours has and what the recommended interval is.

The manufacturer’s manual always takes priority. A generic interval is just a guideline.

The Special Case of Diesel

If you have a diesel car, there are no ignition spark plugs in your engine. Diesel doesn’t ignite the mixture with a spark; it does so through compression.

What a diesel engine does have are glow plugs (also called heaters). Their function is to heat the combustion chamber on cold starts to facilitate ignition. When they fail, the car starts poorly in low temperatures, and a specific warning light may appear on the dashboard (usually a spiral symbol or an orange coil).

Glow plugs have their own service interval, typically between 60,000 and 120,000 km depending on the manufacturer, though they can also fail sooner if driving conditions are demanding.

What Happens If You Ignore Them Too Long

Extending the service life beyond what’s recommended doesn’t always have immediate consequences. But there comes a point where effects accumulate:

Fuel consumption rises progressively because incomplete combustion wastes fuel. Power drops, especially at low revs. And if a spark plug fails completely (what’s called “cylinder misfire”), the catalytic converter can be affected by the unburned hydrocarbon it receives, which can turn a 10-euro part into a repair costing several hundred.

There’s no need to panic. But it also doesn’t make sense to ignore simple, predictable maintenance.

Write Down the Change and Keep Track of It

The problem with spark plugs isn’t the change itself. It’s that, because they last so long, it’s easy to forget when the last one was done.

If you record the date and kilometers every time you do maintenance, you have the information when you need it. In OwnAutoCare you can note the spark plug change with date, kilometers, and cost, and set a manual reminder for the next interval. Without having to rely on the shop remembering for you or searching for the invoice in a drawer.

Spark plugs don’t fail all at once. They wear down slowly. That’s precisely what makes them easy to manage if you keep a minimum of track.