This one is for petrol cars only and covers the whole ignition system. Or rather, the whole modern ignition system; we’re not going to cover ‘points’ here, as they’re only found in much older cars. So, want to know about ignition coil, HT lead and spark plug issues and how to sort them out? This is the page for you.

First up: what are all these components?

Let’s start with the spark plug, as that’s been present and correct in all petrol-powered cars since the day spark ignition was invented. The spark plug is screwed into the cylinder head and the bottom of it (the electrode) protrudes into the combustion chamber of the engine’s cylinder. A strong electrical charge (or spark) jumps across a gap in the spark plug’s electrode causing the fuel and air mixture in the cylinder to ignite, This mini explosion pushes the piston down and turns the crankshaft. This process happens thousands of times every minute.

E3_Spark_Plugs_2_Plug_Image (1) ignition-coil-dmb908 Z037B

The timing of this spark is crucial to an engine’s smooth running, its fuel economy and power output and that’s controlled in different ways depending on the design and the age of the car. Newer models generally have individual ignition coils, one per cylinder, that slide down over the tops of the spark plugs. The timing is controlled centrally by the Engine Control Unit (ECU) and various sensors around the engine and can be varied depending on operating conditions and the engine control strategy.

Volkswagen TFSI engine with individual coil packs

Most older cars feature a single ignition coil, which is responsible for sending a high voltage charge from the car’s electrical system to the spark plugs. Between the coil and the spark plug in this design sits a distributor, which is usually mechanically driven by the engine. This, as the name suggests, distributes the charge to the correct spark plug at the correct time. The spark timing can be adjusted manually by altering the rotation of the distributor. Between the distributor and the spark plugs in this case are HT (High Tension) leads, also known simply as plug leads.


Honda engine with distributor & HT leads


The two diagrams below show older (on the left) and newer ignition systems. Both do more or less the same thing  just the newer system distributes and times the spark electronically as opposed to mechanically.


Common signs of an ignition system fault:

As the ignition system is integral to how a petrol engine works, if something goes wrong with it, then it’s likely to have a very obvious effect on the engine’s operation. It’s usually the first place to start when diagnosing issues, as it’s often relatively easy to get to the root of the problem.

Reduced performance or increased fuel consumption:

All ignition systems gradually deteriorate so it’s a good idea to keep an eye on your car’s fuel economy and make a record of it. A degradation in this area over time indicates that the ignition system may need a little work. Spark plugs would often the first component to check in this case. The chart below shows what a normal and worn spark plug should look like – as well as a whole host of other conditions worth knowing about. We have an in-depth article about spark plugs and how they work if you want to know more.


Poor starting:

If the car is harder to start than before (and the battery is in good health), then it could be that a coil is on the way out, or the spark plugs need to be replaced.

Misfiring / rough running / stalling:

Misfiring when running could be accompanied by black smoke and smell of unburned fuel coming out of the exhaust. This happens when a spark plug or plugs don’t fire for some reason, and the fuel/air mixture in the cylinder exits the exhaust without being burnt. This means worse performance and potentially a really rough engine running. It’s possible for one or multiple cylinders to misfire consistently, which many people refer to as ‘missing’. This causes a characteristic roughness in the engine and horrible vibrations through the car, particularly at low engine speeds. It may also cause the engine to stall at idle or when coming to a stop. If you have a constant misfire you will also have a noticeable lack of power.

This video shows a car misfiring which will give you an idea of what it sounds like:

Misfires due to failing ignition components can be sneaky though and may not be present 100% of the time, making them harder to diagnose – for example a misfire due to a failing coil pack may only be noticeable under full load eg hard acceleration but may perform perfectly at idle or part throttle.


While the ignition system may not be wholly responsible for all of the above symptoms, it’s a good place to start, certainly with older cars. With modern models, many garages will plug the car into a diagnostic computer first and foremost. This should give a fault code to help identify precisely the issue. That’s the theory anyway… It isn’t always that simple, unfortunately, and a good mechanic is one that doesn’t solely rely on such methods and is able to troubleshoot the issue without resorting to simply replacing everything and hoping for the best.

Replacing the ignition components.

Checking that spark plugs are receiving voltage is not difficult to do, but as high voltage is involved, caution needs to be excercised. It’s really a job best left to the competent DIY’er or trained mechanic. Once upon a time, the spark plugs were replaced at every service, but that has changed in recent years as the car manufacturers demand longer service intervals. The result is that most spark plugs are now long life items. Mechanics once had to adjust the spark plug gap too using a feeler gauge, but now they are usually supplied with the correct gap for the car. Nevertheless, when buying spark plugs, it’s important to pay attention to the technical specifications and make sure the items you’re buying are suitable for your car. For those that drive a little harder or spend a lot of time on the road you may want to invest in high-performance spark plugs, often marketed as ‘Platinum’ or ‘Iridium’ as they generally last longer and withstand sustained higher temperatures particularly well.

Changing your spark plugs

We’ve made a couple of videos demonstrating how to change spark plugs and ignition coils. The first video below shows a slightly older ignition system design with a single ignition coil and HT leads going to each of the 4 spark plugs. The car we used in this demo was a Ford Focus with 1.4 Zetec engine.

The second video shows a typical modern set up featuring multiple ignition coil packs which fit directly onto the spark plugs. The car we used in this demo was our mk5 Volkswagen #Project GTi:

Changing your spark plugs is a relatively easy DIY job on most cars (there are, as always some exceptions – Hi Subaru owners!) but there are a few things to be aware of. First up, make sure the engine is off and, for safety, take the keys out of the ignition and put them somewhere safe. To be on the safe-side it’s always a good idea to disconnect the batteries’ negative terminal and cover it with a glove or rag to eliminate any risk of it sparking or shorting off anything. Whether a car has on-plug coils or traditional HT leads, the job of removing them from the tops of the plugs is fairly similar. There’s a rubber/plastic cover that grips the cylinder head and the top of the spark plug. This may be firmly attached so apply reasonable pressure to release it, grabbing it as close to the spark plug as possible so as not to damage the lead itself. With this removed, you should see the top of the spark plug.

A long spark plug socket with rubber insert is needed and long straight extension. Loosen the plug using a ratchet.socket-detail Old plugs can be extremely tight so use a long ratchet for leverage and keep it upright or you risk breaking the plug. If you do, extract any debris before you remove the plug itself, as it could be disaster for the engine if it falls down into the cylinder. Indeed, it’s worth blowing any dust and material from around the plug out before loosening it too. Once the plug is out, quickly replace it with a new one, making sure nothing falls down the hole into the cylinder. Make sure you start the thread turning by hand to reduce the chance of cross-threading.

The plugs need to be tightened to the correct torque setting for your particular engine (you’ll find it in the owners manual), but beware that most modern cylinder heads are made of aluminium, so over tightening can damage the threads or even crack the head itself – so be careful!! Replace the HT lead or coil ensuring they’re properly seated back into position. Once all the plugs are changed, start the car up to ensure it is running correctly and take it for a drive. Replacing a HT lead or modern coil pack can sometimes be done by hand without any tools at all! Older cars with single coils, distributors and ignition modules will certainly require some basic hand tools.

A top tip is to photograph the engine bay and get close ups of the components you’re about to work on before you take it apart so you can double check how it all goes back together. Better still, invest in a workshop manual for your car if you do this sort of thing regularly.

And finally…

While you may put off checking why your engine misfires every now and then and you think you can live with reduced fuel economy and deteriorated performance, it’s worth bearing in mind that a constant and persistent misfire can actually damage the the engine’s cylinder linings and pistons – so don’t put it off for too long or the repair bill could end up growing dramatically!

Most of the components mentioned in this article are relatively inexpensive to buy and fit. Click the links below to browse MicksGarage for ignition parts for your car:

All Ignition system parts

Spark plugs

HT/Ignition leads

Distributor caps

Ignition coils