| Sign | Reasons/Features |
| A. The insulator tip is light gray to gray in color | Spark plugs with the prescribed heat rating are installed. The engine (fuel-air mixture preparation, ignition system) operates as prescribed. Mechanical wear of the engine is within acceptable limits |
| B. The tip of the insulator is whitish in color | Spark plug insulator overheats. Inappropriate fuel used. Reduced valve clearance. Exhaust gas composition needs to be measured. Check engine operation (fuel-air mixture preparation, ignition system), fuel-air mixture probably too lean. Risk of severe engine damage (piston scuffing and burning, exhaust valve burning) |
| C. The spark plug is dark in color and the insulator is black | Spark plugs with too high a heat rating are installed. Spark plugs are worn out. High proportion of short-distance trips. Dirty air filter. Engine operation is impaired (air-fuel mixture preparation, ignition system). Oxygen sensor operation is impaired. Exhaust gases are black |
| 0. The spark plug looks black and the insulator is oily | See C. Defective spark plug wire or spark plug cap. Broken oil scraper ring. Piston scoring. Worn valve train gasket. High oil consumption. Blue/black exhaust gases |