I've had two codes in my Engine Control Module
0303 and 0304. After a visit to a dealer (Nissan
of Natick, MA) was told that I need Oxigen censor
recall and a new knock sensor. (O2 sensor is under
recall, btw)

With regards to the latter and since I had some
doubts about spending 400 dollars in labor
to replace a $20(?) sensor, I was told that
the sensor has an "open" and it's a "hard
failure" which will cause the "engine check light to stay on".

Neither of this statments are evidently true:
After resetting the ECM, the engine check light
stays OFF. When I measured the resistance of the
knock sensor, it checks at 551 KOhms, which I
think is well within spec. So the dealer seems
to be scamming me... BUT, when I check ECM for
codes, the 0304 code, which is Knock sensor still
flashes. What am I missing