How to Read Your Spark Plugs

Schedule Spark Plug Replacement
Service Area

$96.47 - $642.49

Price range for all vehicles
Average rating from 3,580 customers who received a Spark Plugs Replacement.

Spark plugs can provide valuable information about your vehicle’s performance and can predict potential problems. Learning how to read a spark plug is quick and easy, and can equip you with the skills to determine when to change out your spark plugs for optimal performance.

In short, the reading of a spark plug involves evaluating the condition and color of the tip of the spark plug. Most often, a light tan color around the tip of the spark plug indicates a healthy and well performing engine. If the tip of the plug is any other color or condition, it indicates an issue with either the engine, the fuel system, or the ignition. Follow the steps below to learn how to read your car’s spark plug.

view of several different spark plug failures

Part 1 of 1: Inspecting the condition of your spark plugs

Materials Needed

removing spark plug

Step 1: Remove the spark plugs. Refer to your vehicle service manual to determine the location of your spark plugs, how many there are, as well as instructions on pulling them out.

Depending on your vehicle, you may need a ratchet socket drive wrench and an extension bar to remove your spark plugs. Examine your spark plugs, comparing them to the diagram above, to familiarize yourself with the plug’s condition and the engine performance.

  • Warning: If you were running your car prior to spark plug inspection, let your engine fully cool. Your spark plugs could still be extremely hot, so be sure to leave ample cooling time. Sometimes a plug will bind in the cylinder head if the engine is too warm during removal.

  • Tip: Remove and read one spark plug before moving on to the next, as having too many out of the engine at once can create confusion later on. If you choose to put old spark plugs back in, they will need to go back into their respective places.

black soot insulator

Step 2: Check for soot. When you first begin to examine a spark plug, check for any black soot on the insulator or even the central electrode.

Any soot or carbon build up indicates the engine is running rich on fuel. Simply readjust the carburetor to achieve a full burn or have the problem diagnosed for you. Then no more soot or carbon deposits should get on the insulator nose of any of the spark plugs.

spark plug white deposits

Step 2: Check for white deposits. Any white deposits (often ashy looking) on the insulator or central electrode often indicates excessive oil consumption or fuel additives.

If you notice any white deposits on any spark plug’s insulator, check your vehicle's valve guide seals, piston oil control rings, and cylinders for issues, or have a qualified mechanic diagnose and repair the leak.

spark plug tan blisters

Step 4: Check for white or tan blisters. Any white or light tan blisters with a bubbling look to them can indicate an issue with the fuel or the use of fuel additives.

Try using a different gas station and different fuel if you tend to use the same gas station.

If you do so and are still noticing blisters, check for a vacuum leak or have a qualified mechanic do so.

spark plug with black peppering over plug

Step 5: Check for black spots. Small, pepper-like black spots on the nose of a spark plug can indicate light detonation.

When this condition is severe, it is also indicated by cracks or chipping in the plug’s insulator. Additionally, this is an issue that can damage your intake valves, cylinders, rings and pistons.

Double check that you are using the type of spark plugs with the correct heat range recommended for your vehicle, and that your fuel is at the correct octane level recommended for your engine.

If you notice that the spark plugs you are using are not within the correct heat range for your vehicle, you should have your spark plugs replaced as soon as possible.

old and new plug

Step 6: Change your spark plugs regularly. To discern whether a plug is old or newer, examine their center electrode.

The center electrode will be worn down or rounded if the plug is too old, which can then cause your vehicle to misfire and have difficulty starting.

Worn down spark plugs also prevent the car from reaching its optimum fuel economy.

If old plugs are left unreplaced for long enough, damage can be done throughout the ignition system. If you are uncomfortable replacing your spark plugs yourself, or aren’t sure which spark plugs to use, consult with a qualified mechanic to determine the best course of action. If you need a replacement of your spark plugs, a technician from YourMechanic can come to your home or office to perform this service for you.

For more on spark plugs, you can also read our articles “How to Buy Good Quality Spark Plugs,” “How Long Do Spark Plugs Last,” “Are there Different Kinds of Spark Plugs,” and “Symptoms of Bad or Failing Spark Plugs.”


Next Step

Schedule Spark Plug Replacement

The most popular service booked by readers of this article is Spark Plug Replacement. Yourmechanic’s technicians bring the dealership to you by performing this job at your home or office 7 days a week between 7AM-9PM. We currently cover over 10k cities and have 100k+ 5-star reviews... LEARN MORE

SEE PRICING & SCHEDULING

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Recent Spark Plug Replacement reviews

Excellent Rating


YourMechanic Spark Plug Replacement Service

Average Rating

4.9/5

Number of Reviews

3,580

Rating Summary
3,368
127
16
15
54
3,368
127
16
15
54

Luis

16 years of experience
183 reviews
Luis
16 years of experience
Dodge Challenger - Spark Plugs - Wallington, New Jersey
I just wish I knew about this service earlier. I would highly recommend yourmechanic and specifically Luis. I saved so much time not sitting in a dealership.
BMW X1 - Spark Plug Replacement - Linden, New Jersey
Luis was awesome! He was early to the appointment and super knowledgeable about the work. He was also really communicative, which I liked. Great personality and professionalism. Would definitely book again. Actually, I did book again for the follow up maintenance. 5 stars

Eduardo

17 years of experience
330 reviews
Eduardo
17 years of experience
Mercedes-Benz E550 - Spark Plugs - Missouri City, Texas
Showed up right on time and did a great job. Really courteous as well, we will be asking for Eduardo next time we need our cars serviced.
Jeep Grand Cherokee - Spark Plugs - Houston, Texas
Eduardo was great! Will definitely use him again. Arrived on time and finished much faster than was quoted. Unfortunately, I ordered the parts that the site said I would need, but several of them were not needed. And they can't be returned to Amazon. Thus I am out $70+ on unused parts. Of course that's not Eduardo's fault at all! I just want to recommend that other customers buy their parts locally so they can be returned!

Chris

14 years of experience
233 reviews
Chris
14 years of experience
Ford Thunderbird - Spark Plugs - San Diego, California
Very nice and knowledgeable mechanic. We were trouble shooting a 1955 Ford Thunderbird that evidently have been been flooded in its Holley teapot carb. We also checked for spark in all the plugs and cylinder just to be sure it wasn't a spark issue. In the end, we managed to "clear the flood" in the teapot and got her running again! Thanks Chris.

Chuck

8 years of experience
112 reviews
Chuck
8 years of experience
Volkswagen Jetta - Spark Plugs - Dallas, Texas
Chuck is very professional and expert in his job and I strongly recommend you to select him in your next car service!

Need Help With Your Car?

Our certified mobile mechanics make house calls in over 2,000 U.S. cities. Fast, free online quotes for your car repair.

GET A QUOTE