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How To Tell If A Spark Plug Is Bad


Clean, working spark plugs are essential to keeping your vehicle running smoothly and reliably. These tiny powerhouses are catalysts for everything your vehicle does, from bringing the engine roaring to life when you start your car to burning fuel efficiently as it moves down the road. 


As with any vehicle component, your spark plugs will need service and replacement eventually, usually between 30,000 - 50,000 miles. Mounted at the top of your motor's cylinders, the spark plug is what ignites the fuel and oxygen mixture inside your engine's cylinders, exploding it, driving the piston down, and repeatedly creating power thousands of times per minute to put your car in motion.


Over time, this process can leave spark plug electrodes dirty and less efficient. Dirty or clogged air filters, driving at low speeds for extended periods of time, and having an improper fuel/air mixture inside your engine can all cause premature wear that leads to bad spark plugs. 


Spark Plug What does a bad spark plug look like


If you notice any of the following vehicle issues, they may be bad spark plug symptoms.


Poor Fuel Mileage: As your spark plugs become dirty or worn, they may not burn fuel as efficiently. Their ability to ignite the fuel and air inside your motor decreases, and a full fuel tank may not get you as far as it used to. If you see a drop in your miles per gallon, dirty or damaged spark plugs may be the culprit. 


Difficulty Starting Your Engine: Fouled spark plug electrodes may have difficulty igniting the fuel mixture as well, which can lead to difficulty starting your engine. If you've noticed your engine cranks well, but has difficulty starting, bad spark plugs may be the culprit. 


Rough Idling: If you notice shaking or bouncing while your engine is running at idle speeds, this can be caused by bad spark plugs or spark plug wires. When spark plugs begin to fail, they burn fuel at an inconsistent rate, which can lead to noticeable engine struggles at low speeds.


Engine Misfiring: While driving, bad spark plugs may result in a motor that feels like it's hesitating when you step on the gas or that jerks suddenly. This could be due to the spark plugs igniting fuel inconsistently and will also result in poor fuel mileage. 


While spark plugs are critical to the operation of your vehicle, the good news is that replacing them is relatively easy and affordable. If you've experienced any of the bad spark plug symptoms above or suspect that your spark plugs need to be checked, visit your local AAA car care center for a free inspection and quote for repair.