| sign | Reasons / features |
| A. The tip of the insulator is light gray to gray in color | Installed spark plugs with the prescribed glow rating. Engine (air-fuel mixture preparation, ignition system) works as prescribed. Engine mechanical wear within acceptable limits |
| B. Whitish insulator tip | The spark plug insulator is overheating. The wrong fuel is being used. Reduced valve clearance. The composition of the exhaust gases must be measured. Check engine operation (air-fuel mixture preparation, ignition system), probably too lean air-fuel mixture. There is a risk of severe engine damage (scuffing and burnout of pistons, combustion of exhaust valves) |
| C. The appearance of the candle is dark, and the insulator is black | Spark plugs with a too high glow number have been installed. Worn out spark plugs. A high share of trips over short distances. The air filter is dirty. Engine malfunction (air-fuel mixture preparation, ignition system). The oxygen sensor is malfunctioning. Black exhaust gases |
| 0. The appearance of the candle is black, and the insulator is oily | See C. Faulty high voltage ignition wire or spark plug tip. Broken oil ring. Piston seizure. Worn valve train gasket. High oil consumption. Exhaust gases blue/black |
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