Engine Misfire 2026: Symptoms, Causes & How to Fix

An engine misfire occurs when the combustion process in one or more cylinders is interrupted, the fuel either fails to ignite or ignites at the wrong time. You might hear a loud "pop" from under the bonnet, notice rough idling, or feel a sudden loss of power while driving. It can happen to any petrol or diesel car, old or new, and the causes range from a worn-out spark plug costing a few pounds to a blown head gasket requiring serious repair work. Understanding why your car is misfiring and knowing when to investigate yourself versus when to take it to a professional can save time, money and stress. Below are the most common causes of engine misfires, the symptoms to watch for, and what to do next.

Common causes of engine misfire

Engine misfires broadly fall into three categories: ignition problems, fuel system issues and mechanical engine faults. Not every judder or jerk you feel actually comes from the engine. Transmission trouble, clutch wear and even sticking brake pads can mimic the sensation of a misfire. The sections below cover each cause in turn so you can narrow down the problem or at least give your technician a head start.

Spark plugs and ignition components

The ignition system generates the spark that ignites the air-fuel mixture inside each cylinder. If there is no spark, there is no ignition, and the cylinder misfires. Ignition parts are relatively inexpensive and straightforward to inspect, so they are the logical starting point for any misfire diagnosis.

Spark plugs, first thing to check

Spark plugs are the most common culprit behind an engine misfire. Over time, electrodes erode, carbon deposits build up and the gap between electrodes widens, all of which weaken the spark. Spark plugs are cheap and, on many cars, accessible enough for a confident home mechanic to inspect or replace. If you have intermittent rough running, check your spark plugs first.

Faulty plug wires and ignition misfires

Plug wires, also called HT leads, carry high-voltage current from the ignition coil to each spark plug. When they degrade through heat, vibration or age, you may notice a cluster of tell-tale symptoms:

• Erratic or rough idling
• An engine misfire under load or at speed
• The check-engine light illuminating on the dashboard
• Interference on the car radio
• A noticeable drop in fuel economy

Wear and tear on ignition components can also produce a loud popping sound as combustion is interrupted mid-cycle. If you hear that pop, book a diagnostic appointment, as continuing to drive risks damaging the catalytic converter or other engine components.

Fuel system problems

Even with a healthy spark, the engine needs the right amount of fuel delivered at the right time. Two common fuel-related issues can trigger a misfire.

Clogged fuel injectors

Fuel injectors spray a precise mist of fuel into each cylinder. Over time, and particularly if you do a lot of short journeys, deposits can build up and partially block the nozzle. The affected cylinder then receives too little fuel and misfires. As a first step, you can try adding a quality fuel-injector cleaner to your tank. If the misfire persists, the injectors may need professional ultrasonic cleaning or replacement.

Lean misfire (air/fuel ratio)

A lean misfire happens when there is too much air relative to fuel in the combustion chamber. You will often notice it most at idle, and it may clear once engine speed rises. Several components can be at fault:

• A weak or failing fuel pump
• A blocked fuel filter
• A sticking or faulty EGR valve
• A leaking intake manifold gasket

Diagnosing the exact source of a lean condition requires specialist equipment, so this is generally a job for a qualified technician.

Mechanical engine faults

When ignition and fuel delivery are both healthy, the misfire may point to a deeper mechanical problem inside the engine itself.

Worn piston rings

Piston rings seal the gap between each piston and the cylinder wall, helping regulate oil pressure and maintain compression. When they wear, pistons can move out of alignment and firing timing suffers. Common signs of worn piston rings include excessive smoke or fumes from the exhaust and reduced power when accelerating. This is a major repair and should be carried out by a professional workshop.

Leaking or blown head gasket

The head gasket sits between the engine block and the cylinder head, sealing combustion gases, coolant and oil into their respective channels. If the gasket fails between two cylinders, compression can leak from one into the other, lowering pressure in both and causing rough, uneven idling. A blown head gasket often also causes coolant loss, white exhaust smoke or a milky residue under the oil filler cap. Prompt repair is essential to avoid serious engine damage.

When it isn't the engine, driveline and brakes that feel like a misfire

Not every stutter originates under the bonnet. Before assuming the worst, consider whether the sensation matches any of the following.

Transmission issues

Erratic performance, particularly hesitation or jerking when the car changes gear, can feel remarkably similar to a misfire. If the rough running only happens during gear changes or under specific load conditions, a transmission fault may be to blame. A professional diagnostic scan is the quickest way to distinguish between an engine misfire and a gearbox problem.

Chattering clutch and related clutch problems

On manual cars, a chattering or juddering clutch can produce a misfire-like vibration, especially when pulling away from a standstill or in overdrive. The root cause is often worn clutch discs or a failing pressure plate. If you feel the judder through the clutch pedal rather than through the engine note, the clutch is the more likely suspect.

Sticking brake pads and warped rotors

Jerking when you slow down does not necessarily point to the engine at all. Sticking brake pads, warped brake discs or a rough transmission downshift can all produce a similar sensation. If the symptom only appears during deceleration or braking, have your brakes and transmission inspected before investigating the engine.

Quick checks you can safely do (and what must be done by a technician)

There are a handful of things you can investigate at home before booking the car in:

• Visual spark plug check — if you are confident removing a spark plug, look for heavy carbon deposits, oil fouling or a worn electrode.
• Listen carefully — note whether you hear popping, spluttering or rough idling, and whether it happens at idle, under acceleration or both.
• Record when symptoms occur — misfires only at idle may suggest a lean condition; misfires under load may point to ignition or fuel delivery.
• Try an injector cleaner — if you suspect dirty injectors, a single tank treatment with a quality fuel-system cleaner is a low-cost first step.

When to book a service and who can help

If your DIY checks have not resolved the problem, or if the misfire is severe, loud popping, significant power loss, or a persistent check-engine light, get professional help. Many service centres have access to manufacturer-level diagnostic equipment. Booking an early diagnostic can prevent further damage and reduce overall repair costs. Technicians provide diagnostics and estimates so you can decide on repairs.

Arnold Clark has over 150 service centres across the UK, staffed by trained technicians with access to manufacturer-level diagnostic equipment.

Summary: quick checklist

• Check spark plugs first, they are the most common and cheapest cause of a misfire.
• Suspect ignition wiring if symptoms include radio interference, a check-engine light or erratic idling.
• Consider brakes or transmission if the judder only happens when slowing down or changing gear.
• Never tackle electrical diagnostics yourself, leave these to a qualified technician.

FAQ

1. What is a misfire on a car?

   A misfire is an interruption of the combustion process inside one or more engine cylinders. You may hear a loud pop, feel the engine run roughly at idle, or notice a drop in power; causes include ignition, fuel or mechanical faults.

2. What are the most common causes of engine misfires?

   The most frequent causes are worn spark plugs, degraded ignition wiring, clogged fuel injectors, an air/fuel imbalance (lean misfire), worn piston rings and a blown head gasket. In some cases, what feels like a misfire is actually caused by transmission problems, a chattering clutch or sticking brake pads.

3. Can I drive with a misfire?

   If the misfire is mild and intermittent, you can usually drive carefully to a garage, avoiding long journeys or heavy acceleration. If the misfire is severe, loud popping, major power loss or a flashing engine warning light, stop driving as soon as it is safe to do so and arrange urgent diagnostics from a qualified technician; continuing to drive risks damaging the catalytic converter and other costly components.