Part explained
Replacing spark plugs: interval and symptoms
Spark plugs are among the cheapest maintenance parts and among the most neglected. A set costs less than a tank of fuel, yet worn plugs cause higher consumption, worse acceleration, and over time damage the catalytic converter. This page covers the interval, the symptoms, and which type belongs in which engine.
By the onderdelen.autos editorial team · Updated
What do spark plugs do?
Spark plugs ignite the air-fuel mixture in each cylinder of a petrol engine. The plug delivers a spark across a precisely set electrode gap. Wear erodes the electrode, widens the gap, and increases the voltage required to arc — eventually causing misfires.
Signs of worn plugs
- Idle is rougher than before.
- Hesitation under acceleration.
- Higher fuel consumption (a 10% rise is realistic).
- "Misfire detected, cylinder X" via the OBD-II reader.
- Misfires in wet weather (moisture on coils or plugs).
When to replace
Type-dependent: standard copper plugs at 30,000 km, platinum at 60,000, iridium and dual-iridium at 90,000 – 100,000. The exact interval is in the owner's manual. Always replace the whole set together — a set is cheap and even wear gives the most stable running.
Typical interval: 30,000–90,000 km
What does a set cost?
Standard plugs: € 8 – € 20 for a set of four. Iridium: € 25 – € 60 per set. Workshop labour: 0.5 – 1 hour on accessible engines, 1 – 2 hours when coils or intake parts must be removed.
| Condition | From | To |
|---|---|---|
| New | € 8 | € 40 |
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DIY?
Difficulty: Easy
About an hour for most cars. Important: torque to the manufacturer's spec — too tight cracks the ceramic body, too loose leaks compression. Use anti-seize only when the manufacturer specifies it.
- Spark plug socket (often 16 or 21 mm with rubber insert)
- Torque wrench with low range (12 – 30 Nm)
- A feeler gauge — only to verify factory gap; most modern plugs come correctly set
Frequently asked questions
Can I upgrade from copper to iridium?
Often, yes — provided the iridium variant exists in the same heat range. Longer service life is the trade-off against higher up-front cost, usually a net saving over time.
What is the heat range of a plug?
How much heat the plug shifts to the cylinder head. "Hotter" plugs retain more heat (good for light loads, bad for sustained sport), "colder" the opposite. Stick to manufacturer spec.
Per cylinder or all at once?
All at once. Even wear gives the most stable ignition.
Does the interval apply to diesels?
Diesels use glow plugs (cold-start only), not spark plugs. Glow plugs have their own interval, usually 100,000 – 150,000 km, replaced reactively on fault codes.
What do I do with a "misfire" fault code?
Swap the plug from the affected cylinder with another — does the code move? Then it's the plug. If not: the coil or lead.
Related parts
Spark plugs from Dutch inventory
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