Generally, before your spark plugs need to be replaced, your vehicle is going to give you some type of warning. Here are the first three warnings most vehicles give off when their spark plugs need to be replaced:

It's Difficult To Start Your Vehicle

You should be able to insert your key into your ignition, turn it over, and have your vehicle start right up. The first sign you'll notice when your spark plugs start to fail is that you have to turn your key more than once to get your vehicle to started.

If your vehicle will not start, it is usually because your battery needs to be replaced or your spark plugs need to be replaced. You can use a voltmeter to test your car's battery and make sure that it isn't behind your car's starting problems. If your battery is okay, then your spark plugs are giving you a heads-up that they are going out. If you don't replace them immediately, one day when you turn your key in the ignition, it will not start at all. 

Your Vehicle Outputs Too Much Energy When It Is Idling

When you are stopped at a light and your vehicle is idling, your revolutions per minute, or rpm, should be relatively low. You can see your rpm on your dash on the left side; they are designations of a 1000. When your vehicle is idling, your engine should not be spinning very much. Most vehicles idle at or below 1000 rpm.

If your vehicle is pushing over the 1000 mark when you are idling, your engine is having to exert too much energy to stay at rest. This generally occurs when your spark plugs are getting old and are not able to help out your engine like they should. 

Your Engine Turns Off And Back On

Another way your spark plugs let you know they are getting old is by turning off your engine. Your vehicle may start up okay, but after running for a few minutes, your engine may turn off briefly. Your engine may restart on its own, or you may have to turn the keys in the ignition again to get your vehicle going.

This is more commonly referred to as your engine "misfiring." The cylinders in your engine are not getting the power they need because your spark plugs are old. When some vehicles misfire, they make a really loud sound, like a boom. If you feel your vehicle turn off and back on, or hear it make a loud boom, you need to check out your spark plugs. 

If you notice your vehicle idling at high rpm, or if you're having difficulty starting your engine, and your engine is misfiring, you need to either replace the spark plugs in your engine yourself, or take it into your mechanic for an auto repair. If you continue to wait to fix your spark plugs, your engine may start surging when you are driving, which can lead to a serious accident.