A lot can be determined from analyzing a spark plug from a vehicle. A normal spark plug will appear grayish-brown on the electrode and ground strap. This indicates that the fuel system and ignition system are properly tuned.
Timing belts are made from rubber and high tensile fibers. The cambelt synchronizes the camshaft and crankshaft timing. It is important to understand that this belt is different than the serpentine belts. If the belt breaks, it can cause serious engine damage.
Most manufacturers do not specify a time that a timing chain will need to be replaced; chains should normally last the lifetime of the engine, but unfortunately due to a lack of oil changes, manufacturing faults and general wear and tear, sometimes engine chains will need to be replaced.
The oxygen sensor measures the amount of oxygen traveling through the exhaust system. More oxygen in the exhaust stream means a lean fuel mixture, less means a rich mixture. With this information, the ECU is able to fine tune the air/fuel ratio of the engine.