What is Engine Misfire? Causes, Symptoms and Fixes

Ever felt your car hesitate, stumble, or shake like it suddenly forgot how to be a car? That’s an engine misfire. One or more cylinders stop firing properly, power drops, the ride gets rough and your fuel economy nosedives. You might even hear a pop or feel the whole car rock at idle. 

Misfires usually happen because air, fuel or spark aren’t doing their job. Worn spark plugs, clogged injectors, faulty coils, vacuum leaks and low-quality fuel are the main culprits. 

Symptoms vary from shaky acceleration to a flashing check engine light, but the story’s always the same. Something essential has gone wrong in the combustion process.

Fixes can be as simple as replacing a plug or coil pack, topping up fuel quality, or cleaning carbon deposits. Other times, you’re staring down deeper issues like compression loss or timing failures.

In today’s article, I’ll answer it all. I’ll walk you through what misfires actually are, why they happen, the signs to look out for and whether fixing it is worth the money or if it’s time to scrap the thing entirely.

What causes an engine misfire?

A misfire happens when the air, fuel and spark inside a cylinder fail to ignite at the right moment. Most misfires trace back to issues in one of five areas: ignition, fuel delivery, air intake, compression or engine management sensors.

Ignition system problems

Your spark plugs and ignition coils are responsible for lighting the fuel. If a plug is worn out or a coil is weak, that spark becomes unreliable. The engine stumbles because one cylinder stops pulling its weight.

This is one of the most common problems, but also one of the cheapest fixes. New plugs or a fresh coil pack can make a night-and-day difference.

Fuel system problems

The engine needs a precise amount of fuel at a precise moment. Blocked injectors, a tired fuel pump or poor-quality fuel can mess with that balance. When the engine gets the wrong amount, the burn becomes uneven. You press the accelerator and instead of pulling cleanly, the car hesitates or jerks because one cylinder is basically taking a timeout.

Air intake problems

Your engine needs a clean, steady supply of air to mix with fuel. If the intake manifold has a leak or the air filter is clogged, the airflow becomes uneven or restricted. That throws the fuel mix off balance and makes combustion patchy.

Common signs of this include a rough idle, flat spots when accelerating and a louder hiss from under the bonnet if air is escaping where it’s not supposed to. Fixing the leak or replacing a dirty filter usually brings the engine back to a smooth, healthy burn.

Compression issues

Each cylinder has to seal tightly so the air-fuel mixture can be squeezed before it ignites. If pressure leaks out through worn piston rings, damaged valves, or a failing head gasket, the cylinder can’t produce enough power and the engine starts missing. These faults usually cause a constant shake and, unfortunately, often come with higher repair bills.

Sensor failures

Modern engines rely on sensors to make rapid, continuous adjustments to fuel delivery, ignition timing, air-to-fuel ratio, all of it.

Common culprits include:

• Oxygen (O2) sensor
• Mass airflow (MAF) sensor
• Manifold absolute pressure (MAP) sensor
• Crankshaft position sensor
• Camshaft position sensor
• Knock sensor
• Engine coolant temperature (ECT) sensor
• Throttle position sensor (TPS) sensor

If a camshaft or crankshaft position sensor starts failing, the engine will mistime the spark. If the oxygen sensor misreads the exhaust gases, the ECU will add too much or too little fuel. Either way, combustion becomes unstable and misfires happen as a result.

If this is the issue, the obvious symptom is the check engine light. But you’ll also probably notice poor throttle response, or irregular revs. The upside? Sensor faults are usually easier to pinpoint with diagnostics and, compared to mechanical failures, much cheaper to sort.

Symptoms of an engine misfire

When your engine misfires, you’ll usually feel it straight away. The car will stumble, shake, hesitate or lose power when you accelerate or sit at idle. Most misfires cause a noticeable drop in performance and if left unchecked will trigger warning lights and damage other engine components over time.

Whether it’s a one-off hiccup or a constant rough idle, a misfire always points to a problem with combustion.

Rough idle and acceleration issues

One of the first things most drivers notice is a lumpy or uneven idle. The engine may feel like it’s pulsing or shaking slightly when you’re stopped at a red light, almost like it’s running off-balance.

As you press the accelerator, the power delivery might hesitate, surge or feel flat (especially if the misfire is happening under load). This is a sign one or more cylinders aren’t pulling their weight.

Check engine light

The check engine light doesn’t always mean something bad. In fact, it could mean dozens of things, ranging from as miniscule as a loose gas cap to as catastrophic as a catalytic converter failure. But if it’s flashing or comes on alongside rough running, chances are good it’s a misfire. 

Modern ECUs are quick to pick up when a cylinder isn’t firing properly. They’ll store a fault code — P0300 for random/multiple cylinder misfires or P0301-P0308 when the misfire corresponds to a specific cylinder (1 through 8). To warn you, it’ll light up the dashboard.

A solid light might mean a minor issue, but a flashing one usually signals active misfiring that could damage the catalytic converter if you keep driving. Either way, it’s worth getting the codes read sooner rather than later.

Loss of power and stalling

When one or more of your engine’s cylinders can’t ignite the proper air-fuel mixture, it’ll cause a noticeable drop in engine power when you accelerate, climb hills or drive at motorway speeds. In more severe cases, the car might jerk or even stall altogether. This is particularly common at low speeds or when you’re coming to a stop.

The reason for this is that one or more cylinders are failing to contribute, which throws off the engine’s balance and forces it to work harder with less. If the stalling becomes frequent, it’s a sign the misfire’s getting worse.

Decreased fuel efficiency

When a cylinder misfires, the ECU responds by adjusting the fuelling to try and compensate, normally by injecting more fuel than needed. At the same time, any unburnt fuel from the misfiring cylinder goes straight out through the exhaust, which wastes energy and overworking the catalytic converter.

The engine ends up doing more work with less result, which means you burn through fuel faster. So if, along with the abovementioned symptoms, your MPG drops suddenly and you haven’t really changed your driving habits, a misfire is one of the first things worth checking.

Unusual engine vibrations or noises

When one cylinder stops firing properly, the engine’s rhythm breaks down. That imbalance causes noticeable vibrations. You might feel the whole car shaking slightly, or hear irregular popping, coughing, or sputtering sounds from under the bonnet or through the exhaust.

These symptoms go hand-in-hand with rough idling, poor acceleration and stalling because they’re a byproduct of the exact same issues. They only become more pronounced over time, which is why you need to get to the garage ASAP if you notice them.

How to fix engine misfire?

You fix an engine misfire first by identifying what’s causing the cylinder to stop firing properly, then replacing, repairing or cleaning the part that’s responsible. That could be as simple as swapping out a spark plug or as complicated as rebuilding part of the engine. 

The key is to diagnose the fault correctly. Most modern cars will log a fault code that tells you which cylinder is misfiring and in many cases, a garage can trace the issue in one diagnostic session.

Once you’ve addressed the root cause, the misfire usually disappears straight away.

Fixing ignition system issues

If the misfire is caused by a weak or inconsistent spark, the first place to look is the ignition system. Start by inspecting the spark plugs. If they’re worn, fouled or cracked, they’re not going to fire reliably. Replacing them is often enough to solve the issue.

If that doesn’t fix it, move on to the ignition coils or leads. A failing coil can cause intermittent misfires in one or more cylinders and it’s not always obvious until a mechanic tests it or swaps it out.

In most cases, ignition-related faults are straightforward to diagnose and relatively cheap to repair, which is why they’re usually the first things a mechanic will check.

Repairing sensor failures

Modern engines rely heavily on accurate sensor data to run smoothly. If just one key sensor starts feeding the ECU bad info, the whole combustion process falls apart. 

To fix it, most garages will start by scanning the ECU for fault codes. This usually points straight to the faulty sensor. From there, the solution is generally a direct replacement. Crankshaft and camshaft sensors are common failure points, as are mass airflow sensors that get coated in grime over time.

Addressing fuel system problems

Fuel-related misfires usually come down to one of three things: not enough fuel, too much fuel or poor-quality fuel. A clogged injector blocks fuel from reaching the cylinder, while a failing fuel pump or dirty filter reduces overall pressure across the system. On the flip side, if the injector is leaking or stuck open, it floods the cylinder and throws off combustion completely.

The fix depends on the cause:

• Cleaning or replacing injectors
• Swapping the fuel filter
• Or testing fuel pressure to check the pump

If you’ve recently filled up with poor fuel, running a tank of higher-quality petrol with an injector cleaner additive sometimes helps. But if the misfire keeps returning under load or at higher RPMs, it’s worth having the fuel system properly inspected.

A quick note here: A lot of dealers and garages will try to upsell a generic “fuel system service” during your regular maintenance for somewhere between £100 and £150. Most of the time, it’s not worth it.

The only real exception is for direct injection engines (like VW’s TSI or BMW’s N20), where carbon can build up on the intake valves over time. A proper induction cleaning (not just a fuel additive) actually makes a difference there.

But if your car uses port or multipoint injection (most modern gas cars do), that kind of carbon build-up isn’t a concern since the fuel itself helps keep the valves clean. Unless you’re having a clear issue, skip the upsell.

Fixing engine compression issues

If your engine misfires constantly, burns oil or coolant or smokes heavily from the exhaust, that’s your cue to get it professionally checked. Compression loss won’t fix itself.

Compression problems usually aren’t DIY territory but here’s how a mechanic will approach it:

1. Run a compression test or leak-down test to identify which cylinder(s) have low pressure.
2. Check for causes. It could be worn piston rings, burnt valves, or a blown head gasket are the usual suspects.
3. Inspect internally if you need to. This might involve removing the cylinder head to visually confirm damage.
4. Repair or rebuild. This could be anything from a valve job to a full engine rebuild, depending on the severity.

Costs vary massively. A simple head gasket replacement might land around £500 to £1,000. Valve work or piston ring replacement can push well over £1,500. And if you’re into full rebuild territory, expect £2,000 to £4,000+ or more for modern turbocharged engines.

Cost to fix an engine misfire

The cost to fix an engine misfire in the UK can range from as little as £40 for a faulty spark plug or coil pack to over £2,000 to fix worn internal components or serious compression loss.

Here’s what that looks like at a glance:

• Simple fixes (plugs, coils, sensors): £100 to £250
• Fuel system work: £150 to £400
• Compression repairs: £600 to ~£2,500
• Engine rebuilds: £2,500 to £5,000+
• Full engine replacement: £1,500 to £10,000+

In addition to regional labour rates and the car you drive, the final price comes down to the root cause.

Cost of replacing spark plugs and ignition coils

Here’s a breakdown of what you’re typically looking at in the UK when replacing spark plugs and ignition coils, plus what drives the cost up or down:

• Spark plugs: For most vehicles you’ll be looking at £100 to £250 all in (parts, labour, diagnostics) for a standard petrol car.
• Ignition coils: According to our data, the average cost is around £150, but can range from £70 to £220 depending on the vehicle and complexity.

A basic small petrol engine (say a 1.0 to 1.4 litre) will be cheaper. Larger engines, luxury brands (e.g. BMW, Mercedes) and harder‑to‑access engines (turbocharged, direct injection) push costs higher. For example, spark plug replacement for a BMW 3 Series might run as high as £217.

Note: Choosing OEM parts will cost more, but I strongly recommend them. Aftermarket equivalents may lower the parts bill but you’ll want to verify their quality and whether or not they void your warranty.

Fuel injector replacement costs

On average in the UK, replacing a fuel injector runs around £120 to £160 per unit.

Depending on accessibility, engine complexity and the number of cylinders, labour can push the bill higher. I’ve seen quoted totals exceed £350 to £400 for one injector on certain models. And if multiple injectors are at fault (common on older/high‑mileage engines), the cost scales up significantly.

Condition of other components also plays a role. If fuel system damage (e.g. due to leaking or degraded injectors) has impacted other parts like the fuel rails and sensors), you’ll pay more.

That said, replacing faulty injectors ensures each cylinder gets the correct amount of fuel at the right time. That restores smooth combustion, which means:

• Power returns (you’ll feel the difference when accelerating).
• Rough idles and misfires disappear.
• Fuel economy improves because you’re not dumping unused fuel or doing inefficient burns.
• Emissions and engine stress reduce, helping other components stay healthy longer.
  So it’s well worth the investment.

Repairing engine compression issues

Here’s a quick breakdown of typical costs in the UK when you’re dealing with compression‑related misfires (hate to break it to you, they’re a lot higher):

• Minor repair (e.g., replacement of one leaking valve + head gasket reseal): £600  to  £1,500.
• Moderate repair (multiple cylinders, valve job + piston rings): £1,000 to ~ £2,500 (average piston‐ring replacement quoted £877.06 in the UK) 
• Major repair or semi‑engine rebuild (full cylinder head off, pistons, rings, liners + labour): £2,500  to  £5,000+.
• Full engine rebuild or replacement (when compression loss is extreme or the block is damaged): £1,500  to  £10,000 depending on make/model and whether the engine is new or reconditioned.

If a cylinder isn’t sealing, it’ll cost you more than a plug change. Early detection keeps the bill lower and ignoring the risks will eventually land you in full rebuild territory.

Cost of professional diagnostics and repairs

The only way to get an accurate figure is through proper diagnosis. Guessing usually leads to wasted time and money.

A lot of garages charge for diagnostic service before repairing or replacing anything because it takes time and specialised equipment. In the UK you’re typically looking at £30 to £100 just to diagnose what’s causing the misfire.

Beyond the diagnosis fee:

• If the fault turns out to be simple (spark plug, coil pack), repair costs stay relatively low.
• If it’s more complex (fuel injectors, compression test required, major internal work) the labour, parts and time start to add up.
• Diagnostics might reveal deeper engine damage; at that point the repair bill will include both the diagnostic fee and significant parts/labour.

Before you start, ask the garage technician: “What is included in the diagnostic fee?” and make sure you get a detailed quote before any work begins.

Is it worth fixing an engine misfire?

In a great deal of cases, fixing an engine misfire is absolutely worth it. The majority of them are caused by relatively simple faults like worn spark plugs, ignition coils and blocked injectors. These are quick fixes that restore performance, fuel economy and reliability back to normal.

But if the issue points to serious internal damage and your car is already near the end of its life, scrapping is the smarter move.

When is repairing worth it?

If the plugs, coils, sensors or injectors cause the misfire, you’re only looking at a few hundred pounds or less. That’s always worth it (unless the car’s 10+ years old, has other problems and is almost completely worthless).

Even moderate repairs like valve replacements or a gasket fix are still worth it if the car is in otherwise good condition and holds value. And if you’ve recently invested in tyres, brakes or servicing, repairing the misfire helps protect those costs.

Fixing it early also avoids knock-on damage to the catalytic converter or engine internals.

When scrapping your car is the better option

If diagnostics reveal major compression issues, bottom-end engine wear or a failed head gasket, repairs quickly rise into £1,500 to £3,000 or more. For older and high-mileage vehicles with little resale value, that kind of spend doesn’t make sense.

So if the car is already unreliable, needs multiple repairs or has MOT failures stacking up, it’s probably time to call it. In that case, scrapping is the cleaner exit and sometimes, surprisingly, the scrap value comes close to the repair quote.

You can find out your car’s scrap and reusable parts value by getting a free scrap car quote with us. It takes 30 seconds and if you accept, our collection team will pick your car up today or tomorrow and pay you on the spot.

What are the tips to prevent engine misfires?

Preventing engine misfires comes down to staying on top of basic maintenance. Most misfires don’t happen out of the blue. They build up slowly as components wear out, clog up or get ignored.

If you keep your ignition, fuel and air systems in good condition, you massively reduce the chances of one. Not to mention, you protect your engine and keep your running costs down.

Keep the ignition system in check.

Worn spark plugs, tired ignition coils and cracked leads are top reasons engines misfire. Stick to your service intervals for spark plug replacement and don’t ignore the signs of coil pack failure (e.g. intermittent misfires or hesitation under load). If one coil fails, it’s smart to replace the full set, especially if your car has a lot of miles on it.

Of course, use high-quality plugs that match your car’s spec. Cheap or mismatched ones will throw off the spark timing and strength, which leads to weak combustion or potentially no ignition at all.

Maintain a clean fuel system.

Use the highest-quality fuel whenever you possibly can to prevent clogging and damage to the fuel system. A blocked fuel filter or weak fuel pump causes low fuel pressure, which starves the injectors and leads to misfires. And never let your tank run to empty; that’s when dirt gets pulled into the system.

Consider using a top-rated fuel injector cleaner every few thousand miles if your engine is known for carbon build-up. And on direct injection cars, professional intake cleaning every 40,000 to 60,000 miles makes a big difference in fuel pressure and engine efficiency.

Check air filters and sensors regularly.

A clogged air filter chokes the engine and throws off the fuel-air mix. Replace it at the recommended interval, or sooner if you drive in dusty places or dense urban areas.

Keep an eye on sensors like the MAF and oxygen sensor, too. If they fail, they won’t always throw a warning light right away but they can quietly cause fuelling errors that lead to misfires.

You can carefully clean a MAF sensor using proper MAF cleaner spray. Just remove it, spray the wire or film and let it dry fully before refitting. Never touch it or use anything abrasive. 

Recalibration usually isn’t needed if you’re just cleaning, but if the sensor’s been replaced, your car may reset the readings automatically after a few driving cycles. Or, it might need a quick ECU reset with a scan tool.

Drive smoothly and responsibly.

Driving style plays a bigger role in long-term engine health than most people seem to realise. Hard acceleration, constant stop-start traffic or lugging the engine in too high a gear all increase the wear on your car’s ignition and fuel components. Warm the engine properly before driving hard, avoid cheap tuning mods and keep your service history up to date.