The knock sensor responds to spark knock caused by Pre-detonation of the Air/Fuel mixture. As the flame front moves out from the spark plug ignition point, pressure waves in the chamber crash into the piston or cylinder walls resulting in a sound known as a knock or ping. This is caused by using a fuel with a low octane rating, overheating, or over advanced timing. Sometimes it can be caused by hot carbon deposits on the piston or cylinder head that raise compression. A knock sensor, which serves to warn of engine knock or pinging due to a bad air/fuel mixture, can go bad. Signs that the knock sensor is bad are a sluggish engine, poor fuel economy and slow acceleration. Most of the time, the knock sensors don't fail entirely but go "soft" and create false signals. You can check the knock sensor through an electrical harness.