How To Check a Spark Plug? (Easy 5 Step Guide)

Want to know how to check a spark plug yourself? We've got the answer!

In this guide, you'll learn:

  • How to check a spark plug
  • Supplies you'll need to check a spark plug
  • How often to check your spark plugs

And much more!

How to Check A Spark Plug?

So, where do you start when figuring out how to get it back running again? By removing and inspecting the spark plug.

Or maybe you've got a check engine light illuminated on the dash of your vehicle. You ran the codes, and you have a misfire on cylinder 'x'. 

It's now time to start the diagnostic process which includes inspecting the condition of the spark plug. Where do you start? Read further for instructions on checking your spark plug(s).

Here are the main reasons why you might want to check the condition of your spark plug(s):

  • Bad fuel economy 
  • Misfiring
  • Slow acceleration
  • The engine is difficult to start
  • Weak spark or no spark
  • Excessive fuel consumption

So, before you check a spark plug, I recommend you read through this quick guide that will help ensure you perform the right procedures for successful spark plug maintenance.

What You Need To Know About Checking a Spark Plug

To check a spark plug, you are going to need a basic understanding of what a fouled spark plug looks like. By comparing an old spark plug side to side with a new one it will be more obvious to identify which spark plug is bad, but don't mistake a slightly used plug for being bad.

When identifying a bad plug, look for signs of carbon buildup or blistering on the electrode tips. 

One of the most obvious signs of a fouled spark plug is when it is caked in engine oil. Be sure to fix the oil leak before installing a new plug to avoid premature failure of the new spark plug. 

Supplies You’ll Need For Checking a Spark Plug

For removing and inspecting a spark plug you will need these tools:

  • Spark plug socket 
  • Pliers
  • Feeler gauges or spark plug gapper tool
  • Basic socket wrench set (for removing coil packs)
  • Flashlight

How To Check a Spark Plug(5 Step Guide)

  1. Remove spark plug lead
  2. Remove spark plug from the cylinder head
  3. Perform inspection
  4. Gap the spark plug to spec
  5. Reinstall the spark plug

Step 1 - Remove Spark Plug Lead

Use a pair of pliers or even your hands to remove the spark plug wire that provides electricity to the spark plug. If you are working on a fuel-injected vehicle, you will have to unplug and unbolt the ignition coil for access to the plugs.

Step 2 - Remove Spark Plug from Cylinder Head

If the spark plug is located deep into the cylinder head you will have to use a spark plug socket to extract it. This type of socket has rubber on the inside that prevents damaging the spark plug upon removal and installation. Shine a flashlight down into each hole for a better view of your work.  For simple engines like lawnmowers, you can sometimes get away with using a crescent wrench.

Step 3 - Perform Inspection

Inspect the spark plug for signs of carbon deposits and fouling on the plug tip. Look for oil on the electrodes and also the threads. If you see signs of corrosion or oil on the electrodes, you can try re-jetting your carburetor and inspecting your distributor cap and rotor. 

In the case of fuel injected automobiles, you will see oil fouling if the valve cover gasket is leaking. Misfiring will also occur in fuel injected vehicles if the coil pack has gone bad. If fouling persists, perform a compression test on the cylinder.

In the worst of circumstances, your engine is misfiring because of a piston ring or valve seal issue. In this case, you may want to leave the work up to a professional. 

Step 4 - Gap the Spark Plug to Spec

Your spark plug can misfire if it is not gapped to the right spec. Use the appropriate feeler gauge or a spark plug gapping tool to create the right amount of space between the side electrode and the center electrode. If you notice your engine idling rough, this can be a sign of spark plugs that are gapped improperly. 

Step 5 - Reinstall the Spark Plug

Blow out the spark plug mounting holes with an air hose to clean any debris or residue that might exist. Carefully reinstall the spark plug by using either your fingers or the spark plug socket, and torque it to spec with a torque wrench. If you are working on a small engine, you can get away with just tightening the spark plug down firm. 

Read More >> How to Gap a Spark Plug

Did you know: If not checked and maintained frequently, bad spark plugs can severely shorten your car engine's lifespan.

Other Valuable Resources on Checking a Spark Plug

To avoid fouling your spark plugs it's important to keep your engine free from oil leaks and your carburetor jetted properly(if your engine has a carburetor). 

When spark plugs are too old or too corroded, you are just going to have to put new plugs in your engine. But before you do, be sure that you have taken care of any underlying oil leaks and ignition problems to avoid immediately fouling your new spark plug. 

Don't get discouraged if cleaning your spark plugs doesn't do the trick. There is always a proper solution for making your engine run, and if you get stumped you can always take your engine to a professional mechanic who will know exactly what to do. 

Read More >> How to Clean a Spark Plug

You Can Perform Tests On Your Ignition System with a Multimeter

You can test your spark plugs for continuity and shortage performing a few simple tests with a multimeter.

If you have a misfire that isn't being caused by a bad spark plug, then you may want to test parts like the fuel injectors and ignition coils for malfunction. You may also want to consider cleaning the injectors if you are working on a higher mileage engine. 

You should now have a better idea of how to check a spark plug. We hope this helped out. Enjoy your knowledge!

Read More >> How to Remove a Broken Spark Plug

About Your Mechanic

About Ryan Nichols

Ryan here! My pro mechanic career began as a technician for Mercedes-Benz. After two years of that great experience, I went out on my own in both the automotive and construction fields. I've since pulled my share of salvage yard parts and fixed more vehicles than I can count.

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