How to diagnose / replace Spark plug? Faulty spark plugs can be a major cause of poor running and poor fuel economy. They eventually wear out over time and need to be replaced. They are also a good indicator of the general condition of an engine and how it is running, i.e. if they are black and oily it could mean worn piston rings or valve stem oil seals, if they are black and sooty it indicates an over rich fuel mixture which could be a blocked air filter, a faulty temperature sensor causi