How Often Should Spark Plugs Be Changed?

Knowing how often spark plugs should be changed is part of keeping a petrol car running efficiently. Spark plugs are small components, but they have an important job: each one creates the electrical spark that ignites the air and fuel mixture inside a petrol engine.

There is no single replacement interval that applies to every car. The correct timing depends on the vehicle manufacturer's service schedule, the engine design, the type of spark plug fitted and how the car is used. Some plugs are designed to last considerably longer than others, so checking the handbook or service information is more reliable than relying on a general rule.

What do spark plugs do?

A petrol engine needs a controlled spark to burn the air and fuel mixture in each cylinder. The spark plug sits in the cylinder head and delivers that spark at the right time. The engine control system adjusts ignition timing to suit engine speed, load and other operating conditions.

Every spark plug operates in a hot, high-pressure environment. Over time, the electrodes can wear and the gap between them can change. Deposits may also build up on the plug. These changes can make it more difficult for the plug to produce a strong, consistent spark.

Diesel engines do not use spark plugs for normal ignition. They use glow plugs to assist starting in cold conditions, which are a different component with a different maintenance requirement.

How often should spark plugs be changed?

The best answer is the interval specified by the vehicle manufacturer. Depending on the engine and plug material, a replacement interval may be set by mileage, time, or both. Some cars require spark plugs as part of a scheduled service, while others have a separate replacement point in the maintenance schedule.

Older vehicles or cars with conventional nickel-alloy plugs may need more frequent replacement than newer cars fitted with long-life iridium or platinum plugs. However, a long-life plug is not automatically suitable for every engine. The replacement part must match the vehicle manufacturer's specification.

Driving conditions can also affect the condition of spark plugs. Frequent short journeys may prevent the engine from reaching its normal operating temperature, while heavy traffic, repeated stop-start driving and towing can place different demands on the engine. These conditions do not always mean the plugs need changing early, but they make it more important to follow the servicing schedule and investigate any change in how the engine runs.

If you are unsure when the plugs were last replaced, the service history and vehicle handbook are useful starting points. A garage can also check the maintenance information and inspect the plugs as part of a suitable service. Our guide to how often to service your car explains why scheduled maintenance is based on more than mileage alone.

What are the signs of worn or faulty spark plugs?

Worn spark plugs can contribute to several engine symptoms, although they are not the only possible cause. You may notice:

• the engine takes longer to start than usual
• rough idling or a noticeable vibration when stationary
• hesitation when accelerating
• an engine that feels less responsive
• increased fuel consumption
• an engine management warning light

These symptoms can also be linked to ignition coils, fuel system faults, air leaks, sensors or other engine problems. Replacing spark plugs without checking the wider symptoms may not solve the underlying issue, so it is best not to treat a symptom as a definite diagnosis.

A misfiring engine can produce a flashing or illuminated engine management warning light on some vehicles. If the car is running very roughly, losing power or producing unusual smells or smoke, avoid continuing to drive it unnecessarily and arrange professional advice. A persistent misfire may cause further damage to the emissions system, and a sudden loss of engine performance can make the vehicle less predictable in traffic.

Why does the type of spark plug matter?

Spark plugs may look similar, but their heat range, dimensions, electrode design and materials can differ. The heat range describes how readily a plug transfers heat away from its firing end. It needs to suit the engine, otherwise the plug may not operate within the intended temperature range.

Some plugs use precious-metal electrodes to help reduce wear and support a longer service life. That does not mean they should be fitted as an upgrade without checking compatibility. The correct specification, plug gap and tightening method are important, and modern ignition systems may be sensitive to the wrong part.

Plugs are also removed and inspected for clues about engine operation. Deposits or unusual wear may suggest an issue elsewhere, but interpreting their condition requires care. A single plug can look different from the others for several reasons, so visual inspection alone is not always enough to identify the cause.

Are spark plugs included in a car service?

It depends on the service plan and the manufacturer's schedule. A service may include checking the replacement interval and condition of the plugs, while actual replacement may be due only at a specified mileage or age. A basic inspection is not necessarily the same as fitting new plugs.

When a replacement is due, a technician can fit the correct parts and check related components such as the ignition coils, plug leads where fitted and surrounding connections. This is particularly useful if the car has been difficult to start or has developed a misfire.

If you are comparing different types of maintenance, our guide to what is included in a full car service may help. You can also read about the difference between an interim service and a full service.

Can I change spark plugs myself?

Changing spark plugs can appear straightforward, but access may be restricted and modern engines can use delicate ignition components. Incorrect parts, damaged threads, incorrect tightening or contamination around the plug can create further problems. Some vehicles also require additional checks after an ignition fault is detected.

Basic owner checks, such as looking at the handbook and noting when the plugs were last replaced, are sensible. Safety-critical or detailed mechanical work is better left to someone with the correct tools and vehicle information. If your car is due a service or has developed starting or running symptoms, Ashton's Garage can check the service requirement and inspect the ignition system as part of appropriate servicing work.

How to avoid missing spark plug replacement

• Keep the service schedule with the car and check both mileage and time intervals.
• Record when spark plugs are replaced and which specification was fitted.
• Do not assume a long-life plug can be used without checking compatibility.
• Tell the garage about starting problems, hesitation, rough running or warning lights.
• Arrange an inspection if the engine is misfiring or the vehicle is losing power.

In short, spark plugs should be changed according to the vehicle manufacturer's schedule, not simply when a problem becomes obvious. The interval can vary widely between petrol engines, and symptoms such as rough running may have several possible causes. Keeping the plugs up to date is a small part of good maintenance, but checking the correct schedule helps protect starting performance, smooth running and the rest of the ignition system.