Knock sensors are designed to detect detonation in an engine. Detonation, unlike combustion, is when an explosion occurs inside the combustion chamber that is unplanned. Typically this will occur with poor (low octane) quality gasoline, carbon buildup in the cyinders, or timing that is too advanced. The knock sensor detects these detonations and then sends a signal to the ECU, which either adjusts the engine's ignition or illuminates the 'Check Engine' light.
When a knock sensor fails, the engine will not properly time itself and will usually run pretty rough. Over time this rough operation can damage an engine, so having the knock sesnor replaced as soon as possible is a good idea. Causes of failure are often attributed to other engine problems (such as abnormally high temperatures) or cracked wiring.
Depending on where the knock sensor is whill greatly affect the difficulty of replacing one. Consult with an automotive professional if you lack the necessary information to perform this repair on your own.
We stock and sell a wide variety of quality knock sensors from these manufacturers: