A spark plug is a small, almost invisible part, yet it affects almost everything you feel when you drive a petrol car. When it is working properly, the engine runs smoothly, responds the moment you touch the throttle, and burns only the fuel it should. When it is worn, all of that slowly loses its edge, so slowly that most drivers do not even notice until things get bad. Spark plugs are one of the parts that get forgotten at service time, and one of the parts drivers most easily buy wrong on their own. Here is how to tell when you need new ones, how to pick the right ones, and why cars running on LPG need extra attention.
What a Spark Plug Actually Does
A spark plug has one job: to produce a spark at exactly the right moment so the air and fuel mixture inside the cylinder ignites. That spark repeats dozens of times per second, on every cylinder, every time the engine turns over. When everything is in order, the spark is strong, the timing is precise, and the mixture burns cleanly. The engine is quiet, pulls normally, does not drink extra fuel, and does not push unburnt fuel into the exhaust.
As the electrodes wear down, the spark gets weaker and less reliable. The mixture does not always ignite the same way, some cylinders produce a bit less power than others, and sometimes the plug simply fails to fire. The engine computer tries to compensate by adding more fuel and shifting the timing, and all of that bleeds through into how the car feels, how much it drinks, and whether the check engine light comes on. A spark plug that looks almost new can actually be worn enough to cause problems, because we are talking about a gap measured in fractions of a millimetre.
Symptoms of Worn Spark Plugs You Can Spot Yourself
Most spark plug problems announce themselves before things get serious. If you notice two or more of the symptoms below together, there is a good chance the plugs are involved.
- Engine shakes at idle. When one cylinder runs weaker than the others, the engine loses its rhythm and starts shaking while you sit still. You feel the vibration in the steering wheel and the gear lever. This is not always purely down to the plugs, as we covered in why your car shakes at idle, but it is one of the first things we check.
- Loss of power on acceleration. When you press the gas, the engine does not respond the way you are used to. It feels as if the car is swimming, as if something is pulling it back.
- Higher fuel consumption. A weak spark means incomplete combustion, and incomplete combustion means the engine is burning more fuel for the same result. If your consumption has crept up for no obvious reason, spark plugs are worth checking, as we explain in more detail in our article on higher fuel consumption.
- Hard starting, especially cold. A cold engine needs a stronger, cleaner spark to ignite fuel that is less eager to evaporate. A worn plug is the first to say "not today".
- Misfires. You feel a misfire as a jolt while driving, especially under load or when cruising on the motorway. The engine computer almost always logs a misfire code and turns on the check engine light.
- Check engine light. It does not always mean spark plugs, but very often it does. If the light is flashing, do not keep driving as usual. Flashing and steady lights mean different things, as we explain in the article on the check engine light and whether you can drive.
- Random hesitation with no obvious cause. When a plug misfires sometimes, not always, you get the feeling of "fine one minute, strange the next". Drivers often describe it as "it feels like it hangs up somewhere".
None of these symptoms point only at spark plugs. Coils, plug leads, injectors, the lambda sensor, even a dirty fuel filter can cause the same issues. That is why we never replace spark plugs blindly. We match the symptom to the actual condition of the electrodes and to the diagnostic reading.
When to Replace Spark Plugs and What Types Exist
The exact mileage depends first on the plug type and then on the specific engine. We will not throw numbers at you that you might remember and apply to your car, because the difference between manufacturers can be large. Instead, here is how this picture looks in practice.
Copper (classic) spark plugs have the shortest service life. They are cheap, they work well while they work, but the electrodes wear relatively quickly. Today you mostly find them on older engines and wherever the manufacturer specifically calls for that type.
Platinum spark plugs last significantly longer because platinum erodes more slowly than copper. Their service life is in the middle range, and you will find them as the factory choice on plenty of petrol engines from the 2000s onward.
Iridium spark plugs have the longest interval. The electrode is very thin and made of a material that practically does not wear at normal operating temperatures, so the spark stays consistent for longer. Most modern engines come with iridium plugs from the factory. They cost more per piece, but divided by the kilometres they cover, they are usually no more expensive than copper.
The real answer to "when should I replace them" lives in your car's service manual. If the manual is lost somewhere or you do not feel like digging through it, bring the car to the workshop. We check which type is fitted, how many kilometres they have done, and whether the computer has started logging small misfire events. Replacing them before they become a problem is cheaper than replacing them after they damage the catalytic converter.
How to Choose the Right Spark Plugs for Your Car
This is the part where drivers most often slip up, so let me be clear. The question is not "which brand of spark plug is best", but "which one is right for this engine". The engine manufacturer specified exact specs that include the heat range, thread size, thread length, number of electrodes, gap, and electrode material. That spec is not a marketing suggestion, it is part of how the engine is calculated to work. The wrong plug can overheat the piston, cause detonation, fail to ignite properly, or just not fit physically the way it should.
Well-known and reliable brands that cover almost every engine are NGK, Denso, Bosch, and Champion. Each of them makes copper, platinum, and iridium variants. There is no single universal "best" model you should be chasing, because the same brand makes plugs in dozens of different specs, each one for a different engine. The fact that your neighbour swears by his NGK iridium plugs does not mean the same ones will suit your car.
So how do you pick the right plugs without turning into a mechanic? You have two clean options:
- Open the service manual and find the exact part number (something like "NGK IFR6T11" or "Bosch FR7DPP30X"). Buy that reference. Do not substitute "an equivalent model" from another brand unless you are certain it really is equivalent.
- Bring the car to the workshop and let us check what is fitted and what belongs there. We have these things on the bench every day and we know which plugs go in which engine without guessing.
A note for owners of VW, Audi, Skoda, and SEAT cars with TSI and TFSI engines. These engines are very sensitive to plug spec, heat range, and gap, and the wrong choice shows up quickly through misfire codes and knocking under load. If you drive something from the VAG family, have a look at our page for Volkswagen service in Banja Luka before buying plugs on your own.
Spark Plugs and LPG: Why It Is Not the Same
This is the part most drivers do not know, and it matters. When a car runs on LPG, the mixture burns at a higher temperature and slower than petrol. That means two things for the spark plug: the electrodes carry more thermal load, and the spark itself needs to be stronger to reliably ignite the LPG mixture. In practice, plugs in cars with an LPG system wear faster than they would on petrol alone. The replacement interval gets shorter, sometimes noticeably, depending on how much time the car spends on gas.
For that reason, on LPG-equipped cars we often recommend either plugs with a slightly cooler heat range or iridium plugs that keep the spark stable for longer under those conditions. Again, you cannot just grab any "iridium" off the shelf. It has to match the exact spec. If an LPG car has started hesitating, throwing misfire codes, or losing power, spark plugs are one of the first things we check, before touching anything on the gas system itself. We wrote more about this in the article on when your car runs badly on LPG.
This is an area where a workshop that handles both mechanics and LPG sees things differently from a pure gas shop or a pure mechanics shop. Half of "LPG problems" actually live in the engine, not in the gas system. Spark plugs are a classic example.
Can You Change Spark Plugs at Home
Honest answer. On older petrol engines with open access, where the plugs are visible as soon as you lift the bonnet and there is one cable per plug, yes. Someone with hand tools and half an hour of free time can do it at home without drama. The torque is forgiving and it is hard to damage anything if you go slowly.
On modern engines the story is different. The plugs sit deep in the cylinder head, often under a plastic cover and coil-on-plug units that sit directly on top of each plug. Access is tight, and on some engines you have to remove the intake manifold or other parts to reach every plug. That is where the risks of DIY start to pile up:
- Over-tightening or under-tightening. A spark plug screwed in too hard can damage the thread in the cylinder head. A damaged head thread is a much more expensive repair than a plug change.
- Pulling coils off by force. On coil-on-plug systems, the coils sometimes stick to the plugs and refuse to come off. Pulling them at an angle breaks plastic and connectors.
- Dirt falling into the cylinder. If you do not clean around the plug before you take it out, every grain of sand or ash that drops into the cylinder becomes a problem.
- Wrong tool. A proper spark plug socket has a rubber insert that holds the plug so it does not drop on the way back in. A regular socket can make you drop the plug down the hole, crack the ceramic, and start a frustrating afternoon.
- Wrong gap on new plugs. Some types come with a preset gap you must not touch, while others have to be checked and adjusted before fitting.
If you are not confident about your specific engine, or if this would be your first time, it is better to pay the workshop. Replacing spark plugs is one of those jobs that goes quickly when you do it every day and gets expensive when it goes wrong. We do it as part of a minor service in Banja Luka, or as a standalone job if you only need plugs and a coil check, through our auto mechanic service.
When to Call Us
If you drive a petrol car and notice any of the signs above, rough idle, power loss, higher fuel use, harder starting, or a check engine light, drop by. We check the plugs, the coils, and the ignition timing, so you know exactly where things stand. If your car runs on LPG as well, this is doubly important, because LPG ruthlessly exposes any weakness in the ignition system. Call Auto Gas Gaga in Banja Luka, book a time, and your car will go back to feeling the way it used to.