The knock sensor retards the ignition timing when its senses engine detonation/pinging-like marbles being thrown in an empty tin can-
November, 9, 2011 AT 8:50 PM
Fault codes never say to replace something. They are set in memory when the Engine Computer is not happy with the value a sensor is reporting or the circuit doesn't pass one of the continually on-going self tests. The codes are meant to direct you to the circuit or system that needs to be diagnosed, that's all. About 50 percent of the time the problem is indeed a defective sensor, but it's just as likely there is a corroded terminal in an electrical connector, a wire is rubbed through and grounding out, or it could even be nothing more than a sensor was unplugged while the ignition switch was turned on. Very often people just replace the sensor related to the code, then have the same problem. That adds to the running problem by introducing a new variable to the computer. No two sensors deliver exactly the same signals so the computer learns the characteristics of the new sensor by comparing it to other known conditions based on other sensor readings. When there's a code in memory, the computer may not learn the characteristics of the new sensor. Instead, it makes decisions based on the old sensor that it thinks is still on the engine.
The knock sensor is tuned to detect spark knock or preignition in a cylinder. When that is detected, the Engine Computer retards ignition timing to reduce that spark knock. Mechanical problems inside the engine can mimic that vibration and be falsely detected as spark knock. That can result in the ignition timing remaining retarded, which results in lower power and higher fuel consumption.
There are multiple codes that can be set related to the knock sensor. When the computer retards ignition timing, it knows the knocking should stop or decrease. If the vibration is still there due to something mechanically wrong in the engine, it may assume the knock sensor is reporting knocking when it knows there shouldn't be any, so it thinks that circuit or sensor has a problem. It COULD set the fault code when there's nothing wrong with the sensor.