Spark plugs are among the few engine components that provide diagnostic information about the engine’s health when they are removed. The condition of the electrode and the insulator on a removed plug — the color, the deposit pattern, the electrode wear — tells a story about combustion temperature, fuel mixture, oil consumption, and the plug’s remaining service life. Reading this information correctly takes about sixty seconds and is one of the more useful inspections available without specialized equipment.

The timing belt connects the crankshaft to the camshaft or camshafts, synchronizing the opening and closing of the engine’s valves with the movement of the pistons. In an interference engine — the design used in the majority of modern cars — the pistons and valves share the same space at different times, with the timing system ensuring they do not occupy it simultaneously. When the timing belt fails, synchronization is lost. The pistons and valves meet. The damage is immediate, severe, and entirely preventable.