How to Tell If Your Spark Plugs Need to Be Replaced

Spark plugs are important maintenance items that should not be overlooked or neglected. They play an important role related to engine performance, fuel mileage and exhaust emission standards. The best way to determine when spark plug replacement is needed is the maintenance schedule for your specific vehicle. The life expectancy of spark plugs will vary widely based on the vehicle and engine designs and spark plug type. If you are not the original owner of the vehicle and unaware of its maintenance history, there are some additional indicators to consider.

Signs to Tell If Your Spark Plugs Need to Be Replaced

1. Visual Inspection

Remove and compare your old spark plugs to a new one. If the tip and/or electrode show excessive wear they should be replaced. Also, look for fuel and oil contamination. If contamination is found, the spark plugs should be replaced. However, additional inspection is needed to determine the cause of the contamination whether it is caused by oil, fuel, or coolant. Failing components within the engine itself can cause oil or coolant contamination. Fuel contamination can be caused by failing components within the fuel, ignition and/or emission systems.

Pro Tip: The following are symptoms that may occur with worn spark plugs. Keep in mind, there may be other issues causing the problem. Be sure and do a complete inspection before replacing any parts.

2. Trouble Starting

If you are experiencing what seems to be longer cranking times, especially on a cold morning, you’ll want to look at your spark plugs and related ignition and fuel system components. Over time, spark plugs develop residue from unburnt gases and oils, which reduces your car’s performance and makes cold-starting more difficult. Tough cold starting is not only bad for your engine, but can also drain the battery and increase starter wear and tear.

3. Rough Idling

If your vehicle’s engine is running rough or noisy when idling, you likely have a problem in your ignition system. Heavy rumbling when waiting at a stoplight or in a drive-thru may indicate your car is overdue for maintenance. It may be a spark plug issue, but it could just as easily be another component like ignition coils causing the problem. Inspect the system and determine if the spark plugs or something like the ignition coils and wires need to be replaced. Make sure that you diagnose the issue before you go about replacing parts, otherwise you could end up wasting time and money.

4. Misfiring

Misfires are hard to miss, as the engine will shake so badly that you’ll feel the vibration throughout the car. Misfiring results in poor performance, reduced fuel economy, increased emissions, and rough idling. Replacing spark plugs may help with this issue but, as above, inspect the system first.

5. Decreased Fuel Mileage

Low gas mileage is a subtler sign of worn spark plugs. Deteriorated spark plugs can account for as much as a 30-percent loss in fuel economy, which you’ll definitely feel at the pump and your wallet. If you’ve checked and determined the fuel mileage has dropped, check your spark plugs and related items for signs of wear.

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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