When to Change Spark Plugs, and What Worn Ones Cost You

Spark plugs are the classic quiet service item. They do not fail so much as fade, and because they fade over tens of thousands of miles, the car you are driving today feels normal compared with the car you drove yesterday.

The result is that plenty of people replace them, notice the car feels noticeably better, and realise they had been driving something slightly worse for a year or more without knowing.

What they do and why they wear

A spark plug delivers the spark that ignites the mixture, thousands of times a minute, in an environment of extreme heat and pressure.

Every one of those sparks erodes the electrode very slightly. As it erodes, the gap widens, and a wider gap needs a higher voltage to jump. Eventually the ignition system is working hard to do what it used to do easily, and on some cylinders occasionally it does not quite manage.

That is what a misfire is. Long before you feel one, the engine has been compensating, and the compensation shows up as fuel you are paying for and power you are not getting.

The interval depends on the metal

This is why advice varies so wildly, and why a number a friend quotes may be irrelevant to your car.

• Copper or nickel plugs: the shortest interval, and the cheapest to buy. Common on older engines.
• Platinum: considerably longer, and the middle ground on price.
• Double platinum and iridium: the longest intervals, and the reason some modern cars go a very long way between changes.
• Engines with a coil per cylinder, direct injection or forced induction often specify their own intervals that do not follow the general pattern.

So the honest answer is that your handbook has the number and no rule of thumb replaces it. What is universally true is that fitting a shorter-life plug than specified means the interval you were told no longer applies.

What worn plugs actually feel like

The symptoms are gradual, which is exactly why they get missed.

• Fuel economy that has drifted worse with no other explanation.
• A slightly rough or uneven idle, most noticeable when stationary with the engine warm.
• Hesitation when accelerating from low revs, or under load going uphill.
• Harder starting, particularly on cold damp mornings.
• An engine management light, once a misfire becomes frequent enough for the car to record it.

Any of those on their own has other causes. Several together, on a car whose plugs are near or past their interval, points fairly clearly.

Why leaving them costs more than the plugs

Worn spark plugs do not only affect how the car runs. They put the rest of the ignition system under strain.

A wider gap demands more voltage, and that extra demand is carried by the ignition coils. Coils are meaningfully more expensive than plugs, and a coil that has spent a year working harder than it should is a coil closer to failing. Replacing plugs on schedule is partly about protecting the components behind them.

There is also the catalytic converter. A sustained misfire sends unburnt fuel into the exhaust, and the converter is not designed to deal with that. It is the same mechanism that makes a flashing engine management light urgent, and it is the reason a cheap fault left alone becomes an expensive one.

Fitting the right specification

Spark plugs are not interchangeable, and two that look identical can behave very differently.

The heat range matters: a plug that runs too cold fouls up with deposits, and one that runs too hot can cause pre-ignition, which is genuinely damaging. The reach, the thread and the gap all have to match too, and on many modern engines the gap is set at manufacture and should not be adjusted.

None of this is a reason to be nervous about the job. It is a reason to buy the plug the manufacturer specifies rather than the one that fits and is cheaper.

Is it a job worth doing yourself?

On some engines it is genuinely straightforward: four coils on top, unbolt, unplug, swap, reassemble. On others the intake manifold has to come off first, or the plugs sit deep in wells that collect water and debris.

Two things that catch people out. Plugs are torqued to a specific figure, and over-tightening into an aluminium head is an expensive mistake, so a torque wrench is not optional. And the well should be blown clean before removing the plug, because anything sitting on top of it drops straight into the cylinder when the plug comes out.

If either of those gives you pause, it is a modest job at a garage and usually quoted as parts plus an hour.

The short version

What the old plugs tell you when they come out

Worth asking to see them, because a used spark plug is a readable record of what has been happening inside that cylinder.

• An even light grey or tan deposit on the tip: the engine is running correctly. This is what you want to see.
• Dry black soot: running rich, which points at the fuel mixture, an air intake restriction or a lot of short cold journeys.
• Wet and oily: oil is reaching the combustion chamber, which is a more significant conversation.
• A white, blistered or eroded tip: running too hot, which can mean the wrong heat range or something more serious.
• One plug markedly different from the other three: whatever is happening is specific to that cylinder, which is a useful head start.

That last case is the most valuable. A single odd spark plug turns a vague running complaint into a known cylinder, and it costs nothing but a look.

Find the interval for your engine and the plug type it specifies, and treat both as the answer rather than a starting point. Replace spark plugs on schedule rather than on symptoms, because the symptoms arrive after the fuel has already been wasted.

And if your car has drifted worse on economy and slightly rougher at idle over the past year, check when the plugs were last done before assuming it is something more interesting.