The main characteristic was a P0330 code. The details stated that the code appeared at 1950rpm, with 56% engine load. This rpm threshold is more or less when the knock sensor circuit is activated. The code appeared so it means there was no signal from the sensor. Either bad completely (little probability) or smacked cable. So there was nothing more to do than have a look at it.
There is also the possibility that timing is retarded when there is no signal from the sensor. This decreases power and increases fuel appetite. In my case I didn't feel any big power loss, probably because the other knock sensor was operational.