There's nothing quite like it for ruining your day. You're driving along, minding your own business, when suddenly the car starts to judder, hesitate, and shake, especially when you try to accelerate. It might feel like you're driving over a cattle grid, even on a smooth road. The engine has lost its rhythm, and it's a deeply unsettling feeling. This symptom is what we in the trade call an 'engine misfire'. In simple terms, one or more of the engine's cylinders isn't firing properly. It's a bit like a rower in a boat suddenly dropping their oar – the whole boat becomes unsteady and loses power. Your engine is the same. When one part of the team stops working, the whole operation feels rough and out of sorts. The good news is that it's a very common issue, and usually, it's not a sign that your engine is about to explode. However, it *is* your car's way of telling you loud and clear that something needs attention. Ignoring it can lead to more serious and expensive problems, so it's best to get to the bottom of it.
What this usually means
At its heart, your engine is a finely-tuned machine that relies on a constant, rhythmic series of controlled explosions. This is the combustion cycle. For each cylinder, it needs three things to happen at exactly the right time: a precise amount of fuel, a precise amount of air, and a strong spark to ignite the mixture. This creates the 'bang' that pushes the piston down and turns the engine. An engine misfire is what happens when this perfect sequence fails in one of the cylinders. The 'bang' doesn't happen, or it happens weakly. When this occurs, the cylinder doesn't produce any power on tha
Most common causes (UK cars)
In my experience, over 90% of misfires are caused by a problem in the ignition system – the bits responsible for making the spark. Here are the usual suspects we see in the workshop every week, from most to least common: * **Worn Spark Plugs:** This is the number one culprit. Spark plugs have a hard life and are a service item, meaning they are designed to be replaced regularly (typically every 20,000 to 60,000 miles). As they wear down, the gap the spark has to jump gets wider, requiring more electrical energy. Eventually, the spark becomes too weak to ignite the fuel properly, causing a misf
What to check yourself before booking in
First things first: **safety**. An engine bay can be a dangerous place, with hot parts, moving belts, and high-voltage electricity. Never attempt any checks with the engine running unless you know exactly what you are doing. For these simple checks, make sure the engine is off and the keys are out of the ignition. There isn't a great deal you can fix yourself without tools and experience, but these few simple checks can sometimes save you a trip to the garage, or at least give the mechanic some useful information. 1. **Check the Fuel Cap:** This sounds too simple to be true, but a loose or fau
Is it safe to keep driving?
This is a question I get asked all the time, and I'll give you the honest, straight answer: **No, it's not a good idea.** You might be able to limp the car to a local garage if you're only a mile or two away, but you should avoid driving it any further than absolutely necessary. There are three main reasons for this. First, and most importantly, is the risk of catastrophic damage to your catalytic converter. As I mentioned, a misfire pumps raw fuel into your hot exhaust. This can destroy the catalytic converter in a matter of minutes. A new set of spark plugs might cost you £150 fitted; a new
What a garage will do to fix it
When you bring your misfiring car to a reputable garage, we'll follow a logical diagnostic process to find the root cause accurately, rather than just guessing and throwing parts at it. First, we'll talk to you. A good mechanic will always listen to the driver. When did it start? Is it worse when the engine is hot or cold? This personal account is valuable. Next comes the diagnostic computer. We'll plug our scanning tool into your car's OBD (On-Board Diagnostics) port. This allows us to talk to the engine's 'brain', the ECU. If the engine management light is on, the ECU will have stored a faul
Typical UK repair costs (2025)
The cost of fixing a misfire can vary quite a bit, as it depends entirely on what part has failed. However, here's a realistic breakdown of what you can expect to pay at a typical independent garage in the UK. First, expect a diagnostic fee. This usually costs between **£60 and £120** and covers the technician's time to plug in the diagnostic equipment, retrieve fault codes, and perform the initial investigation to pinpoint the cause. This is money well spent, as it prevents guesswork. If it's a simple fix, the costs will be: * **Spark Plugs:** These are usually the cheapest solution. A set of
Will an engine misfire cause an MOT failure? — Yes, absolutely. A misfire has a direct impact on your car's emissions, and this is a key part of the MOT test. When a cylinder misfires, unburnt fuel enters the exhaust, which the MOT emissions tester will detect immediately as a high hydrocarbon (HC) reading. Furthermore, if the engine management light is lit up on the dashboard due to the misfire, that is also a reason for an instant MOT failur
Why does my car only seem to misfire when it's cold or wet? — This is a classic symptom that usually points towards an ignition system problem, specifically with insulation. On older cars with HT leads, moisture in the air can provide a path for the high-voltage electricity to 'leak' out from tiny cracks in the leads' rubber coating, earthing on the engine block instead of reaching the spark plug. A similar thing can happen with cracks in an ignition coil's
Will using an injector cleaner additive in my fuel fix a misfire? — Honestly, it's unlikely to fix a persistent misfire. While these 'pour-in-the-tank' additives can sometimes help with very minor injector deposit build-up and improve general running, they are not a silver bullet. If you have a misfire caused by a completely blocked or electrically failed injector, no amount of cleaner will fix it. Think of it as preventative maintenance or a potential solution fo
Bob's Mechanical Repairs — independent family-run garage in Birnam, Dunkeld, Perthshire. Call 01350 727 276 or email bob@bobsmechanicalrepairs.co.uk.