How to Test a Spark Plug

If you drive a car with an internal-combustion engine, you probably know that your vehicle needs a spark to ignite a mixture of air and fuel. As their name suggests, your car’s spark plugs provide that spark. There are bad spark plug symptoms, but if you want to be certain before replacing anything, you need to know how to test a spark plug.

How Spark Plugs Work

Provided it has a gas engine, your vehicle likely has an ignition system with a some of the following components. Diesel engines almost never have spark plugs.

Under the hood, there is a battery. This battery stores an electrical current. When you start the ignition, an electrical current moves into an induction coil. This, in turn, increases the electrical voltage to a usable level for the spark plugs. High-voltage electricity then moves through plug wires and ends up in the spark plugs.

Spark plugs typically have two electrodes that facilitate a spark. When sufficiently high voltage hits these electrodes, a spark jumps across. What constitutes sufficiently high voltage varies, depending on the design of the plug and cylinder compression.

The plug’s spark ignites the air/fuel mixture inside each engine cylinder. Naturally, sparks must occur at the right time, when each cylinder’s piston is in the correct position. When functioning properly, spark plugs allow for a smooth burn of fuel.

Spark plugs also play a role in removing heat from a vehicle’s combustion chamber. To do so, each plug transfers heat into the engine’s cooling system. This requires a delicate balance. That is, the plug must remain sufficiently cool to stop premature ignition while being hot enough not to foul.

Most spark plugs have a code printed on them. While the code varies from manufacturer to manufacturer, it typically tells you the plug’s design, heat rating, thread diameter, and hexagon spanner width. You may be able to determine the distance between electrodes by deciphering the code.

Where to Find Spark Plugs

Today’s vehicles usually have the same number of spark plugs as cylinders. Accordingly, if you drive a six-cylinder vehicle, you probably have six spark plugs. You should know, though, that a few vehicles on the road have more than one plug per cylinder. Even though this design is rare, you should check your vehicle’s owner’s manual to determine how many plugs your car has, just in case.

Before cars became increasingly complex, locating spark plugs was easy. After all, plugs usually sit on top or to the side of the engine. With vacuum hoses, filters, wiring and other components, though, the engine compartment has become tight. Therefore, you may have to move a few parts, like Coil-on-Plug (COP) ignition coils, of the way to access your vehicle’s spark plugs. Often, plugs live beneath the engine cover.

A vehicle repair manual can likely tell you how to access your spark plugs. If the instructions are not clear, you can look at an OEM diagram online. Alternatively, you may want to invest in an aftermarket service manual before working on your car, truck, or SUV. Many manuals give you step-by-step instructions on locating and removing plugs. These instructions typically tell you which components you need to move out of the way.

How to Test a Spark Plug

Having functioning spark plugs is essential to a working engine. After all, a bad spark plug can make your engine run roughly, perform sluggishly, misfire, or stall. If you have a faulty spark plug, your Check Engine Light may illuminate, although it sometimes may not. Accordingly, it is good to know how to check spark plugs.

When checking your vehicle’s spark plugs, be sure to look at every plug. Again, you probably have as many plugs as you have cylinders. If you do not check all plugs, you may miss a problem in one of the cylinders.

After removing the spark plug, visually inspect the plug. The following is a guide that may tell you the health of your plugs:

  • Nothing Noteworthy: If your plugs look like they did when you installed them, everything is probably fine. You can probably reinstall the plug and continue to use it.

  • Red-Coated and Normal: An otherwise normal spark plug may have a red coating. Usually, this coating comes from fuel additives and is nothing to worry about. You can likely continue to use the plug.

  • Structural Damage: If you did not install your spark plugs correctly, they may be sitting too low in the combustion chamber. This can cause structural damage that necessitates replacement of the plug. You may also need to inspect other parts of the engine for signs of damage.

  • Detonation Damage: You can see this damage on the electrodes. This often happens when you do not use sufficiently high octane fuel. A plug with detonation damage requires replacement. You may also want to schedule a tune up.

  • Carbon Buildup: If carbon accumulates on the plug’s electrodes, you may have bad wiring or a leak in the injectors. In addition to replacing the plug, you should inspect the thermostat for damage.
How to read your spark plug’s condition

Shop Testing Equipment

How to Use a Spark Plug Tester

1. Prep & Safety

Make sure to wear gloves and other appropriate safety gear. You will need to access the spark plugs by removing any components on top of them according to the proper procedure for your car. Then, disconnect the plug from it’s lead wire.

2. Connect the Tester to the Lead Wire

Attach the tester’s lead to the plug wire. The long metal end of the tester should fit into the wire just like the end of spark plug did earlier.

3. Connect the Tester to the Lead Wire

Connect the boot-end of the tester to the spark plug by sliding it over the end just like how the plug wire was on it before.

4. Crank the Engine

If the engine starts and the plug is functioning properly, the tester’s light will illuminate. Repeat this process for each plug as necessary.

You may wonder what’s the best tool for testing spark plugs? Because spark plug testers come in a variety of styles, which one is right for you probably depends on personal preference. Fortunately, most types, including in-line spark testers, are affordable. When using a tester, though, be sure you wear safety gloves to avoid a shock. Furthermore, always follow operational instructions carefully.

Staying on Top

Keeping up with spark plug maintenance is essential to your vehicle’s engine’s performance. When you are ready to check your spark plugs, order high-quality tools, and plugs, stop by your local AutoZone. With the right equipment and products, you can set yourself up for success.

Advice, how-to guides, and car care information featured on and AutoZone Advice & How-To’s are presented as helpful resources for general maintenance and automotive repairs from a general perspective only and should be used at your own risk. Information is accurate and true to the best of AutoZone’s knowledge, however, there may be omissions, errors or mistakes.

Be sure to consult your owner’s manual, a repair guide, an AutoZoner at a store near you, or a licensed, professional mechanic for vehicle-specific repair information. Refer to the service manual for specific diagnostic, repair and tool information for your particular vehicle. Always chock your wheels prior to lifting a vehicle. Always disconnect the negative battery cable before servicing an electrical application on the vehicle to protect its electrical circuits in the event that a wire is accidentally pierced or grounded. Use caution when working with automotive batteries. Sulfuric acid is caustic and can burn clothing and skin or cause blindness. Always wear gloves and safety glasses and other personal protection equipment, and work in a well-ventilated area. Should electrolyte get on your body or clothing, neutralize it immediately with a solution of baking soda and water. Do not wear ties or loose clothing when working on your vehicle.

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