Common Causes of Spark Plug Failure


Spark plugs play an important role in car engines. They ignite air and fuel in the combustion chambers of cylinders, and start the car engine. If it is bad or worn out it affects the performance of the car engine, fuel efficiency and leads to starting trouble. Studies show that failure in spark plug is the main reason for more than half of all vehicle breakdowns across the world.

The common causes of the failure are fouling, mechanical damage, overheating and improper installing. There are different types of fouling like carbon fouling, oil fouling and lead fouling. In this article, we will discuss briefly the causes of its failure.

Carbon fouling

Carbon fouling is the most common cause of the failure. Carbon fouling means accumulation of carbon deposits on the spark plug. It happens due to choking, which leads to blocked air filter or malfunctioning of the carburetor. The optimal temperature for spark plug is 450°C to 870°C. If the car is idle for a long time, the temperature drops to less than 450°C. Then, the combustion deposits are not burnt off and develop into thick, soft and black carbon, causing carbon fouling.

Oil fouling

Oil fouling happens because of presence of oil in the combustion chamber. Oil enters in to combustion chamber through piston or walls of the cylinders when they are damaged or worn out. It also happens due to a disproportionate oil to fuel ratio. It can be identified by black, shinny, powdery or flaky substance on the spark plug.

Lead fouling

Lead fouling occurs when lead present in gasoline gets deposited and sticks to the spark plug – lead acts as octane enhancers. So, they are mixed in gasoline. Lead fouling can be identified by the greenly deposits. When there is a high load, voltage leaks through the lead that accumulates on the spark plug and causes misfiring. Gasoline car owners should check and replace it after every 1,200 to 1,800 miles.

Worn out

Usually, the firing end will be worn out or the gap between the electrodes will become wider when used for a long time. In that case, normal voltage may not be sufficient to ignite the combustion chamber and the engine does not start. A worn out spark plug will lead to misfiring during acceleration and also causes starting problem.

Mechanical damage

Spark plug threads can get damaged due to improper handling while installing, breakage of ceramic cover, etc. If damaged it results in voltage leak, overheating and abnormal combustion in the cylinders.


We have mentioned earlier that 450°C to 870°C is the optimal temperature range. Temperature more than 870°C overheats the cylinder and damages or melts the firing-end and electrodes. Overheating also causes abnormal combustion. Many small spots, ash deposits or patches on the electrodes are the signs of overheating.

Improper installation

Over tightening or improper tightening will not seal the spark plug tightly with the socket and causes voltage leakage. Make sure to follow manufacturer’s recommendations while installing it to avoid failure.

By taking care of these things, you prevent spark plug failure. A sensible way is to periodically get your vehicle serviced.