What Does a Bad Spark Plug Look Like? (Easy 5 Step Inspection Guide)

Wondering what a bad spark plug looks like? We've got you covered!

In this ProperMechanic.com guide, you'll learn:

  • What you need to know about bad spark plugs
  • Supplies you'll need for replacing a bad spark plug
  • The steps you'll need to identify a bad spark plug

And much more!

What Does a Bad Spark Plug Look Like?

When inspecting a spark plug, it can sometimes be confusing to identify what a bad plug looks like compared to a good one.

So what exactly does a bad spark plug look like? 

While there are more obvious cases like being caked in oil or an electrode tip that is burned to a blistering mess, there are some more subtle signs that you may need to replace or at least gap and clean your spark plug. 

So, before you do replace your spark plugs, I recommend you read this quick guide about identifying a bad spark plug.

What You Need To Know About a Bad Spark Plug

Identifying a faulty spark plug will help you save time and money when repairing your engine. So what are some of the common symptoms of bad spark plugs? Here are a few:

  • Misfiring
  • Rough idle
  • Slow acceleration
  • Excessive fuel consumption
  • Engine misfires
  • Poor fuel economy

Supplies You’ll Need For Identifying a Bad Spark Plug

With just a few simple tools, you will be able to remove and do a quick visual inspection of your spark plugs. Here's what you'll need:

  • Spark plug socket and socket wrench
  • Basic ratchet/socket set
  • Feeler gauges/spark plug gap tool
  • Torque wrench (for vehicles)
  • Flashlight

How To Know What a Bad Spark Plug Looks Like (5 Step Guide)

  1. Remove the spark plug
  2. Visually inspect the plug  
  3. Test the plug with a multimeter
  4. Set the spark plug gap
  5. Replace or reinstall the existing plug

Step 1: Remove the Spark Plug

Removing the spark plug is a fairly easy task that anyone can do. First, for carburetor engines, you will have to remove the spark plug wire boots. 

Fuel-injected engines have ignition coils bolted on to the terminal. You'll need something like a 10mm socket to quickly unbolt and unplug each ignition coil.

You may also have to remove air intake hoses and other plastic parts.

If you can't see where the spark plugs are, shine a flashlight down into the holes to get a better view.

Step 2: Clean and Visually Inspect the Plug  

After removing, use your eyes to find any obvious signs of spark plug malfunction. Cleaning off the threads and electrodes with a good cleaning agent and a brush will help you to get a better look at the condition of the plug.

You can even sand off the side electrode to help the ground connection. 

A normal plug will have brown, gray, and tan on the side electrode. Any of these colors generally means your spark plug is still good. 

Fouled spark plugs will either be caked in oil, have black carbon deposits, or appear to be burned and blistered with a white side electrode. 

Inspect the white insulator part of the plug for signs of cracking. If the plug is tightened too tight, the ceramic insulator will crack and the engine will misfire. 

Step 3: Test the Plug with a Multimeter

You can test a spark plug for continuity and shorting with a Multimeter. 

First, start by setting the multimeter to measure resistance at a range of 20K Ohms Ω. 

Touch the positive test lead to the top of the plug where the spark plug boot hooks onto, and the negative test lead to the center electrode.

If you are testing a resistor spark plug (all automobiles other than race cars use resistor plugs), then you should get a reading of anywhere between 4-8K Ohms Ω. 

If you are testing a non-resistor plug, you can expect a very low or even no resistance between the terminal and the center electrode. 

The second test is performing a continuity test between the center electrode and the ground. In this case, you will set the multimeter to the horn icon in the Ohms Ω section.

Touch one test lead to the side electrode and the other test lead to the ground (the part where the wrench goes). If the spark plug is good, you should hear the multimeter beep which means that the circuit is continuous. 

The last test you can perform is a shortage test between the ground electrode and the center electrode.

Measure for resistance in Ohms Ω between the terminal and the ground by touching one test lead to where the wire boot hooks on and the other to where the wrench goes on.

The spark plug is good if there is zero resistance between the two contact points. If you get any reading at all this means that there is contact between the terminal and the ground, and you should replace the plug.

Step 4: Set the Spark Plug Gap 

The last step before reinstallation is setting the spark plug gap. Use the appropriate feeler gauge or spark plug gap tool to set the manufacturer recommended distance between the two electrodes.

You can also test the actual spark on small engines as shown in the video at the top of the page. A spark plug that is gapped well should have a blue spark.

Step 5: Replace or Reinstall the Existing Plug

Now you should have an educated decision on whether or not to replace or reinstall the existing plug.

Carefully install the plug into the cylinder head, and tighten firmly. If you are working on a vehicle, it's best to torque the spark plug down to avoid engine damage.

Did you know: If your car is taking longer to accelerate than usual, or even if your car is vibrating, it can be a sign that you have a bad spark plug somewhere.

Other Valuable Resources on Knowing What a Bad Spark Plug Looks Like

Did your check engine light turn on due to a misfire? Don't panic. In most situations, you can handle this with a basic set of tools. 

Sure, there are those situations where you blew a valve and are going to need a new head.

There are also those times when the piston rings go bad, and you will have to either do/pay for an engine rebuild or just scrap the engine altogether. 

But the good news is that replacing parts like spark plugs, ignition coils, O2 sensors, valve gaskets, injectors, distributor caps, and rotors is not so difficult. It usually doesn't take you much effort to get your vehicle back up and running again in a lot of cases.

You can also test parts of the ignition system with a multimeter. 

Sometimes it's not going to matter how hard you try to revive your old plugs; you're going to have to install new plugs.

Websites like Amazon some of the best deals on new spark plugs; iridium and platinum are the highest quality options. Stick with brands like NGK, ACDelco, and Bosch for the best performance. 

And finally, if you are still not finding the answers don't be afraid to take your engine to a local repair shop for diagnosis.

You might find you saved a lot of time letting someone else handle your engine's problems. 

Local shops have the experience to give your combustion engine a great tune-up. 

You have the potential to make your engine purr. Good luck!

Read More >> What Size Socket To Use For a 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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