Friend's '96 I30 with 120,000 miles exhibiting low power(was also having trouble going over 85pmh) and was thowing the 0304 code repeatly even after reset. Tested sensor at the harness connector with ohmmeter and it indicated faulty. Ordered new from from Infiniti of Scottsdale (same part number as Nissan knock sensor) for $123 plus a new harness ($14).
Replaced the sensor and harness today and power is back with no more 0304 code. One thing worth mentioning is that when I removed the old knock sensor, I tested it with the ohmmeter leads directly attached to the sensor and it was intermittently bad. However, the harness was totally bad and shorted.
Based on what several Nissan parts folks have told me, it is also a good idea to replace the knock sensor harness when replacing the knock sensor. They do go bad. So, if you are getting the 0304 code alone, try testing it with the meter connected directly to the knock sensor and not the harness. You may only have a bad harness. In my situation both were bad.

 

Where EXACTLY do you connect the ohmmeter to the harness and how? I'm a newbie, but I keep getting the 0304 and am trying to test before I buy anything else.

 

when you say "harness," do you mean ONLY the plastic connector, or do you mean the connector and wire? If its only the connector, the only thing i could imagine going bad are the metal contacts, which could probably be cleaned off (oxidation removed) with some tuner spray from radio shack?