When to change spark plugs:
1. Engine has a rough idle

Your engine idles when it is a stationary and in this position the engine normally produces around 1000rpm. The sound the engine gives off is constant and smooth but if your spark plugs aren’t performing as they should, your engine will produce a rough and jittery sound while producing larger vibrations through the car. Not having this checked can lead to costly damage being done.
2. Having trouble starting your car?

Many people put their car not starting down to being out of fuel or having a flat battery. One possibility you may overlook is having bad or worn spark plugs. If your spark plugs don’t produce the spark needed to get the vehicle moving, then you’re going nowhere. It is also possible that faulty spark plugs are causing your battery to drain. If so you need to have your battery and spark plugs changed as soon as possible.
3. Your engine misfires

When your engine misfires it causes the vehicle to halt for a fraction of a second and then continues it’s usual movement. This means the vehicle isn’t functioning as smoothly as it should because one or more cylinders aren’t firing properly, which can also lead to higher amounts of emissions.
4. Engine surging

When a vehicle sucks in more air than usual in the combustion process it can cause the vehicle to jerk and then slow down or continually start and stop, which means the vehicles engine is working inefficiently. This is also known as engine hesitation and dangerous situation can arise if this occurs in traffic.
5. High fuel consumption

If your spark plugs have deteriorated you’ll notice that your vehicles fuel economy can decrease by up to 30% due to incomplete combustion. If you notice you’re having to fill up more often than usual it can be caused by deteriorating spark plugs. To get back to your vehicle’s optimum level of fuel consumption all you’ll need to do is have your spark plugs changed.
6. Lack of acceleration

If your vehicle is accelerating poorly it is fairly easy to tell. It feels as if the vehicle doesn’t want to respond when you put your foot down, or it does but not instantly as you’ve become accustomed to. It can also feel as if your vehicle is trying really hard to pull itself along. This ‘sluggishness’ can be easily fixed by having the vehicle serviced with a spark plug change.