Car misfiring: why it happens and what it costs to fix

Spark plugs are a routine wear item on petrol engines, typically requiring replacement every 30,000 to 60,000 miles depending on plug type. A worn spark plug produces a weaker or intermittent spark, which fails to reliably ignite the air-fuel mixture in the cylinder. The result is a partial or complete misfire on the affected cylinder, causing rough running, hesitation under acceleration, and increased fuel consumption as the engine compensates. Because plugs wear gradually, the misfire may be barely noticeable at first and worsen over time rather than appearing suddenly.

Typical repair cost: £50 to £150 for a full set of spark plugs including fitting

Urgency: Not urgent but book soon. Worn plugs waste fuel, stress the ignition coils, and if left too long can cause catalytic converter damage from unburned fuel.

Modern petrol engines use a coil-on-plug system with one dedicated ignition coil for each cylinder. When a single coil fails, the cylinder it serves stops firing entirely, causing a persistent single-cylinder misfire. The ECU detects the misfire immediately and logs a cylinder-specific fault code, which makes diagnosis straightforward with an OBD reader. Failed coils are more common on engines with high mileage or where spark plug replacement has been neglected, as worn plugs demand more from the coil and accelerate its failure.

Typical repair cost: £80 to £200 per coil for replacement including labour

Urgency: Book a garage soon. A failed coil sends raw, unburned fuel into the exhaust and catalytic converter, which can overheat and destroy the catalytic converter if the misfire is sustained.

Fuel injectors are precision components that spray a fine mist of fuel into each cylinder at exactly the right moment. A clogged injector starves the cylinder of fuel, causing a lean misfire; a stuck-open or leaking injector floods the cylinder with excess fuel, causing a rich misfire. Both manifest as rough running and a misfire that is often more pronounced under load. Diesel injectors are particularly susceptible to wear at high mileages, and a failing diesel injector can also cause increased smoke from the exhaust.

Typical repair cost: Injector clean: £80 to £200. Injector replacement: £150 to £400 per injector

Urgency: Book within the week. Running with a faulty injector can damage the catalytic converter and, on diesels, cause cylinder wall wear from fuel washing.

Modern engines use a network of rubber hoses and plastic pipes to route air through the intake system. These hoses crack and perish with age, and if a leak develops downstream of the mass airflow sensor, the ECU does not account for the extra air entering the engine. This creates a lean mixture in one or more cylinders, causing a misfire that is often described as random because it does not always point to a specific cylinder. A hissing or sucking noise from the engine bay and an unusually high or unstable idle speed are classic giveaways.

Typical repair cost: Hose replacement: £50 to £200. Intake manifold gasket replacement: £150 to £350

Urgency: Book soon. Running lean can damage spark plugs and the catalytic converter, and the misfire will worsen as the leak grows.

Every petrol and diesel engine relies on compressing the air-fuel mixture before ignition. If the piston rings are worn or valve seats have receded, the cylinder cannot build sufficient compression and combustion is incomplete or absent. A compression test is needed to confirm this diagnosis. Low compression is typically a symptom of general engine wear rather than a single failed component, and the repair cost reflects the extent of the mechanical work required to restore the engine.

Typical repair cost: Compression test: £40 to £80. Valve seat regrind or valve job: £500 to £1,200. Full engine rebuild: £2,000 or more

Urgency: Get a compression test first to confirm before committing to repair costs. The car may remain driveable but performance and economy will continue to deteriorate.

Filling with the wrong fuel type, such as petrol in a diesel or diesel in a petrol, causes immediate and widespread misfiring across all cylinders as the fuel system cannot function correctly with the wrong fluid. Contaminated fuel from a petrol station, while rare, can cause similar symptoms by affecting the combustion process. The key identifying feature is that the misfire begins immediately after refuelling, distinguishing it from gradual wear-related faults. The fuel system must be drained and flushed professionally, and in serious cases the fuel pump and injectors may also require replacement.

Typical repair cost: Fuel system drain and flush: £200 to £500, plus fuel pump and injector replacement if damaged

Urgency: Stop driving immediately if wrong fuel is suspected. Continuing to run the engine circulates contaminated fuel throughout the system and greatly increases repair costs.