Regards! A thorough physical inspection is essential, in addition to checking the calibration of the electrode clearance. With the multimeter in Ohms measure the resistance between the central electrode and the contact tip of the high voltage cable, it must be less than about 5khoms (some spark plugs have no internal resistance and measure 0homs), if it is greater than 5kohms it is bad. If all this is well, you can place the spark plug outside connected to the high wire and in a good ground point to the spark plug socket and see if it produces a good spark, even and bluish. Some spark plugs perforate their ceramic insulator internally and the spark deflects internally and the spark does not occur on the electrode itself. I hope it helps you!