Spark plug
The spark plug is a small but indispensable component in every petrol engine. It generates the decisive spark that ignites the fuel-air mixture in the combustion chamber and thus starts combustion. Without intact spark plugs, the engine runs rough, uses more fuel or, in the worst case, no longer starts at all. In this guide you will learn everything worth knowing about the spark plug: design and function, the differences between copper, platinum and iridium spark plugs, heat range and electrode gap, typical signs of wear as well as a step-by-step guide to replacement. We also give concrete buying recommendations and show what you need to look out for when selecting the right spark plug for your vehicle.
What is a spark plug and why is it so important?
The spark plug is the heart of the spark-ignition engine's ignition system. It sits in the cylinder head and its electrode projects directly into the combustion chamber. Its task is to convert the high voltage supplied by the ignition coil (typically 10,000 to 30,000 volts) into a jumping spark. This spark ignites the compressed fuel-air mixture at exactly the right moment – controlled by the engine control unit. The spark plug is therefore a highly precise wear part that is exposed to extreme thermal, mechanical and chemical loads: temperatures of over 2,500 °C in the spark channel, pressures of up to 60 bar and aggressive combustion residues. Modern spark plugs must withstand these conditions over tens of thousands of kilometres without losing ignition performance. Defective or heavily worn spark plugs lead to misfires, loss of power, increased consumption and can even damage the catalytic converter in the long term. Timely replacement of the spark plugs is therefore a central part of every vehicle service.
Design and function of a spark plug
The most important components of a spark plug
A spark plug consists of several precisely manufactured components that work together to reliably generate the ignition spark:
- Terminal stud (terminal): The spark plug connector or ignition cable is attached here. It conducts the high voltage to the centre electrode.
- Insulator: Made from high-purity aluminium oxide ceramic, it insulates the centre electrode from the shell and withstands voltages of over 40,000 volts. At the same time it conducts heat from the combustion chamber to the cylinder head.
- Shell: Made of steel and carries the thread with which the spark plug is screwed into the cylinder head. It serves as the earth connection and, via the sealing ring, ensures a gas-tight seal.
- Centre electrode: Usually made of a nickel alloy or with a copper core, and in high-quality spark plugs of platinum or iridium. The spark to the earth electrode is created at its tip.
- Earth electrode: A metal piece fitted to the side that projects into the combustion chamber. The spark jumps between it and the centre electrode. Depending on the type, there may be one or more earth electrodes (e.g. on double or triple electrode spark plugs).
- Sealing ring: Ensures a tight connection between the spark plug and the cylinder head, prevents combustion gases from escaping and dissipates heat.
How the ignition spark is created
The ignition process takes place in fractions of a second: the ignition coil converts the 12-volt on-board voltage into a high voltage of several thousand volts. This voltage is passed via ignition cables and the spark plug connector to the terminal stud of the spark plug. Inside the spark plug the current flows to the centre electrode. Since the air in the electrode gap is a very good insulator, a high electric field strength builds up. As soon as the breakdown voltage is reached (depending on electrode gap, pressure and temperature, typically 5,000–15,000 V), the air ionises and a spark jumps from the centre electrode to the earth electrode. This spark has a temperature of several thousand degrees Celsius and ignites the compressed mixture. The entire process lasts only about 1–2 milliseconds, but on a four-cylinder engine at 3,000 rpm it repeats around 100 times per second – per cylinder and spark plug. A precise spark plug is therefore essential for clean combustion, good emission values and low consumption.
Spark plug types compared: copper, platinum, iridium
The service life and ignition performance of a spark plug depend largely on the material of the centre electrode. There are basically three main types: copper, platinum and iridium spark plugs. Each variant has specific advantages and disadvantages that affect replacement intervals, price and engine behaviour. The following table gives a quick overview:
| Feature | Copper spark plug | Platinum spark plug | Iridium spark plug |
|---|---|---|---|
| Electrode material | Nickel alloy with copper core | Platinum (centre electrode) | Iridium (centre electrode, often with platinum earth electrode) |
| Typical service life | 20,000–30,000 km | 60,000–80,000 km | 80,000–120,000 km |
| Electrode diameter | 2.5–2.8 mm | 1.0–1.5 mm | 0.4–0.7 mm |
| Ignition voltage | higher | medium | lower (better ignition) |
| Typical application | older vehicles, simple engines, small cars | modern petrol engines, medium mileage | high-performance engines, direct injection, vehicles with long replacement intervals |
Copper, platinum or iridium – which is better?
The choice of the right spark plug type depends on the vehicle, engine and the manufacturer's specifications. Copper spark plugs are the least expensive option, but they wear out the fastest, as the relatively thick centre electrode burns away more quickly and the electrode gap increases. They are suitable above all for older vehicles or engines that specify short replacement intervals in the manual. Platinum spark plugs offer a noticeably longer service life and more stable ignition performance, as platinum is more heat-resistant and less prone to wear. They are the standard in many modern vehicles from model year 2000 onwards. Iridium spark plugs are the high-end option: the extremely hard and heat-resistant iridium (melting point above 2,400 °C) allows very thin centre electrodes. This lowers the ignition voltage, makes the spark more precise and the spark plug lasts up to 120,000 km. Especially in engines with direct injection, turbocharging or high specific output, iridium spark plugs are often the first choice – not least because many manufacturers fit them at the factory. Important: do not deviate from the recommended type on your own initiative, as otherwise misfires or even engine damage may result.
Understanding heat range and electrode gap
The heat range: cold, medium or hot?
The heat range of a spark plug describes its ability to dissipate heat from the combustion chamber to the cylinder head. It is determined by the length of the insulator nose – the part that projects into the combustion chamber. A hot spark plug has a long insulator nose and dissipates heat more slowly, so it heats up more during operation. It is suitable for engines with low thermal load (e.g. city traffic, low engine speeds), as it reaches self-cleaning temperature and burns off deposits. A cold spark plug has a short insulator nose and dissipates heat very quickly. It is intended for highly loaded engines (turbo, racing, high engine speeds) in order to avoid overheating and pre-ignition. The heat range is usually given as a number code, e.g. for NGK from 2 (hot) to 12 (cold), for Bosch from 13 (hot) to 2 (cold). Always use the heat range specified by the vehicle manufacturer, as an incorrect spark plug can lead to serious engine damage.
Electrode gap: how to set it correctly
The electrode gap is the gap between the centre electrode and the earth electrode. It determines the length of the ignition spark and influences ignition voltage, flame front and combustion quality. Typical values are between 0.7 and 1.1 mm depending on the vehicle, and on some high-performance engines also 0.5 mm or 1.3 mm. Too large a gap leads to misfires, as the ignition voltage is no longer sufficient. Too small a gap produces a weak spark and incomplete combustion. Modern spark plugs with precious-metal electrodes (platinum, iridium) must generally not be bent, as the fine electrodes can break. Copper spark plugs can be carefully adjusted with a feeler gauge. Always observe the manufacturer's specification in the vehicle handbook. When fitting, the correct electrode gap is already set at the factory – but it is still worth measuring before installation.
Signs of faulty or worn spark plugs
Spark plugs wear over mileage and time. Typical symptoms that indicate worn or faulty spark plugs are:
- Starting difficulties: The engine cranks longer than usual before it starts – especially in cold weather.
- Rough engine running: The engine stutters, judders or runs unevenly at idle because one or more cylinders are not igniting reliably.
- Loss of power: The vehicle accelerates noticeably worse, feels sluggish and has less pulling power.
- Increased fuel consumption: Due to incomplete combustion, consumption often rises by 5–15 %.
- Engine warning light (MIL): The engine control unit detects misfires and stores fault codes – the light comes on or flashes.
- Misfires in the exhaust tract: Unburnt fuel reaches the catalytic converter and ignites there – audible as a misfire or backfire.
- Poor emission values: During the emissions test, increased HC or CO values are noticed.
If one or more of these symptoms occur, you should have the spark plugs checked. Even if the spark plugs still work, a change after the prescribed mileage can be sensible in order to avoid consequential damage. By the way: if the vehicle does not start in Berlin, the battery may be to blame as well as the spark plug – it is always worth checking the state of charge and voltage.
Spark plug change: step-by-step instructions
Tools required
To change the spark plugs you need the following tools and aids:
- Spark plug socket in the appropriate size (usually 16 mm or 21 mm, sometimes 14 mm) with rubber insert or magnet
- Ratchet or torque wrench (recommended: adjustable 10–30 Nm)
- Extension for the spark plug socket
- Spark plug grease or copper paste (optional, depending on the manufacturer's specification)
- Compressed air or soft brush for cleaning the spark plug recess
- Feeler gauge for checking the electrode gap (on copper spark plugs)
- New spark plugs (suitable for the vehicle, ideally original part number)
Step-by-step instructions
- Allow the engine to cool: Never work on a hot engine – risk of burns and risk of thread damage to the cylinder head.
- Pull off the spark plug connector: Always grip the connector itself, not the cable. On newer vehicles with coil-on-plug (COP) ignition, you must unscrew and pull off the ignition coil.
- Clean the spark plug recess: Carefully blow out dirt and dust with compressed air so that nothing falls into the combustion chamber.
- Unscrew the old spark plugs: Fit the spark plug socket with extension and carefully turn the spark plug anticlockwise to remove it. Note the installation position of each spark plug.
- Check the thread: Check the thread in the cylinder head for cleanliness and damage. Clean carefully if necessary.
- Insert the new spark plugs: First screw in the new spark plug by hand to avoid cross-threading. Then tighten it with the torque wrench to the prescribed value (typically 15–30 Nm, depending on thread and manufacturer).
- Refit the spark plug connectors or ignition coils: Make sure they are seated firmly and in the correct order.
- Start the engine and check running: Let the engine run for a few minutes and watch for unusual noises, juddering or warning lights.
Torque and installation tips
The correct tightening torque is decisive in ensuring a good seal and heat dissipation without damaging the thread. Typical guide values for spark plugs with a flat seat are 20–30 Nm, for conical seats 10–20 Nm. Always observe the vehicle manufacturer's specifications or those on the spark plug packaging. Use a calibrated torque wrench where possible. Never overtighten spark plugs – a stripped thread can lead to expensive repairs to the cylinder head. A touch of spark plug grease on the thread (only if approved by the manufacturer) makes later removal easier and prevents corrosion.
Buying spark plugs: what to look out for
When buying new spark plugs, you should not simply reach for the nearest one. What matters is that the spark plug matches your vehicle model, engine and year of manufacture exactly. Only then do the thread, heat range, electrode gap and design match. Pay attention to the following points:
Finding the right spark plug for your vehicle
The safest approach is to use the spark plug part number approved by the vehicle manufacturer. You will find this in the service booklet, on the old spark plug or via online catalogues from spark plug manufacturers (e.g. NGK, Bosch, Denso, Beru). Alternatively, you can determine the correct spark plug using the vehicle data (make, model, engine, year of manufacture, displacement, kW/PS). Also check whether your vehicle is equipped with direct injection, a turbo or a start-stop system – this can influence the type. If you are unsure, our team at Electronicx will be happy to help.
Regardless of the choice of spark plug, it is also worth taking a look at other wear parts: modern vehicles, for example, require functioning TPMS sensors for tyre pressure monitoring – we stock a large selection of TPMS sensors for almost all vehicle models. And if your car is in Hamburg or Munich: we deliver quickly and reliably to all German cities – and of course to other regions as well.
We attach importance to fair prices, fast delivery and competent advice. Deliveries to Austria and other EU countries are possible and are charged for.
Frequently asked questions about spark plugs
How often do spark plugs need to be changed?
The replacement interval depends on the spark plug type and the manufacturer's specifications. Copper spark plugs should be replaced every 20,000–30,000 km, platinum spark plugs every 60,000–80,000 km and iridium spark plugs only after 80,000–120,000 km. If in doubt: always observe the information in your vehicle's service booklet.
Can I change spark plugs myself?
Yes, with some manual skill and the right tools, the change can be carried out yourself on most vehicles. What matters is a cold engine, the correct spark plug socket, a torque wrench and the exactly matching spark plugs. With complicated engines (e.g. V engines with spark plugs that are difficult to access) or if you are unsure, you should visit a workshop.
Which spark plug is the right one for my car?
The correct spark plug is always the one approved by the vehicle manufacturer. It must match exactly in thread, heat range, electrode gap and design. Use the old spark plug, the service booklet or an online catalogue to do this. Provide make, model, engine and year of manufacture to find the matching part number.
How do I recognise defective spark plugs?
Typical signs are starting difficulties, rough engine running, loss of power, increased fuel consumption, misfires and an illuminated engine warning light. A visibly burnt electrode, heavy soot deposits or an enlarged electrode gap are also clear indications. With such symptoms, the spark plugs should be checked and replaced if necessary.
What does the heat range mean on spark plugs?
The heat range indicates how quickly a spark plug dissipates heat from the combustion chamber to the cylinder head. A "hot" spark plug retains more heat and is suitable for engines under low load, while a "cold" spark plug dissipates heat quickly and is intended for engines under high load. Always use the heat range specified by the manufacturer, as an incorrect spark plug can lead to overheating or sooting.
Are iridium spark plugs better than copper spark plugs?
Iridium spark plugs offer a significantly longer service life (up to 120,000 km), more precise ignition and more stable spark formation thanks to the fine centre electrode. However, they are more expensive to buy. For modern engines with direct injection or turbocharging, they are often the first choice. Copper spark plugs are cheaper but must be changed more frequently and are mainly suitable for older vehicles.
About Electronicx GmbH
Electronicx GmbH is your reliable partner for vehicle electronics, car batteries, TPMS sensors and spark plugs. Based in Cleebronn, Baden-Württemberg, we supply private customers and workshops throughout Germany and beyond.
Contact:
Electronicx GmbH
Hindenburgstr. 37A
74389 Cleebronn
Germany
Telephone: +49 7135 7194106
E-mail: [email protected]
Managing Director: Andreas Emschanow
VAT ID No.: DE300176428
HRB 752730 (Stuttgart Local Court)
Deliveries to Austria and other EU countries are possible, but are charged for. If you have any questions about spark plugs, batteries or TPMS sensors, you can reach our support team by telephone or e-mail – we will be happy to help.