Nissan Repair: Knock Sensor, what is a knock sensor, nissan altima

Question
I have a 2000 Nissan Altima and my service light came on.  I'm getting a reading for the knock sensor.  What is a knock sensor and its purpose?  Thanks

Answer
Romona,

The knock sensor provides information to the cars computer about how far to advance timing.  If it is broken, as yours is, the car will go into limp mode to keep from damaging the engine with too much advance.  Too much advance causes pre-detonation and that can crack pistons and damage the piston rod bearings.  All this means is the sensor needs to be replaced or there is a possibility of engine damage and prior to this really bad gas mileage since the computer is running a pre programmed (retarded) program to keep from damaging the engine.