My understanding is that the diagnostic system will not set a code if the knock sensor detects detonation. It will set a code (3-1-2) if the signal sent to the knock sensor, at engine start-up, is not complete. In other words, the system checks if the knock sensor circuit is OK, not if the knock sensor ignition retard or fuel enrichment functions have been activated. There have been posts indicating that an LED arrangement can be wired up so that an LED installed in the cockpit will flash if the knock sensor is activated. If you find a well described version of this set-up, please point me in its direction. Thanks!!