The Definitive Guide on How to Check Spark Plugs

Spark plugs are vital for the internal combustion engine of a motor vehicle. They create sparks by transmitting electrical signals to the combustion chamber and those sparks kindle the air-fuel mixture in the chamber. Without good spark plugs, the fuel won’t burn efficiently and the engine may not start at all. You should know how to check spark plugs and recognize the signs that indicate that it’s time to fix or replace the components.

How to Tell If a Spark Plug Is Bad? Watch out for These Symptoms!

Before the spark plug inspection, you should be aware of the common symptoms that indicate that there is something wrong with these components.

These will be the warning signs from your vehicle when the spark plugs are worn out and need replacement:

Trouble Kicking off the Vehicle – Worn spark plugs could be a reason for having trouble starting the car. But, other components in the ignition system can also cause this problem.

Misfiring Engine – Sporadic popping sound from the engine is the sure sign of a misfire. The probable culprit is a frayed spark plug wire or a damaged plug tip that connects to the wire.

how to tell if a spark plug is bad
You should watch out for the symptoms. (Photo Source: howmechanismworks)

Slow Acceleration – A spark plug is made of several materials that work together to produce the sparks. The plugs don’t work properly when these materials wear out, resulting in slowing the acceleration down.

Low Fuel Economy – A car achieves a better fuel economy when the spark plugs work properly because they help with efficient fuel burning in the combustion chamber. If the plug is faulty, it will affect the fuel-efficiency.

How to Check Spark Plugs: A Detailed Analysis

Well, you first have to know what does a bad spark plug look like before knowing how to check spark plugs. The sure sign of a plug being faulty is a crack or multiple cracks on its porcelain cover. Sometimes, you will find a red coating on the ceramic insulation but it is just the additive buildup from the low-quality unleaded fuel.

Read the following steps if you want to know how to check spark plugs:

Look into the plug tip and side electrode.

If these components turn black, it means that they have been running with too much fuel. It can also indicate leaking injectors or bad wiring. In any case, it is a problem linked to the spark plugs. The combustion process is not running at its natural pace, disrupting the burning rate and cleaning process.

what does a bad spark plug look like
See if any part turns black. (Photo Source: clubfrontier)

Examine the wiring of the plugs.

You can measure the resistance of the spark plug wires with a digital multimeter to see if they are working properly. Each foot of a wire should have a resistance of 10,000 to 15,000 ohms. A higher resistance indicates bad wiring. Even a hairline rupture somewhere can cause a total failure, which will shoot the resistance up to infinity!

Detonation damage.

Such a condition can put the engine timing off-balance. Apart from the detonation, gasoline with a poor octane rating can also cause the same consequence.


How to check if spark plugs are firing? You can be sure of the problem if the side electrode is burnt. It happens when the plug fires too soon, does not give enough time for the fuel to be transmitted into the chamber’s air-fuel mixture. Such a condition requires quick attention because the plugs can fail anytime soon.


Check the sparks.

The color of the sparks can tell you whether the plugs are working or not. Remove a spark plug but keep plugged to the cable and ground it on the frame. Now, start the engine and see if it yields any spark.

Blue sparks mean everything is fine but yellow or no spark means it is time to change the plugs. However, there is a problem with this method. When you are testing a plug outside the combustion chamber, the sparks don’t have to undergo the same pressure that they have to inside the chamber. For this reason, a plug sparking in this way does not guarantee that it will be functional during normal running operation.

Don’t Miss These Things during a Spark Plug Diagnosis!

You already know how to check spark plugs but many people still fail to diagnose because of not missing some tricks. During the inspection, you must do these things:

  • Make sure that you have checked all the plugs and their wiring. Some plugs may look functional but you cannot be sure of it until checking them thoroughly.
  • In most cases, replacing the spark plugs with new ones will solve the problem. Like, you can put everything back in order by changing worn out plugs. But, it won’t fix a worn piston ring or a damaged component in the ignition system. It will be better if you have the car diagnosed by a professional mechanic.