Check engine light

The knock sensor is screwed in the engine block below the intake manifold, and the O2 sensor (Oxygen sensor) is in the exhaust system. Most cars have more than one oxygen sensor. The knock sensor detects "knock" or fuel detonation in the engine, that is fuel that starts to burn without spark, and at the wrong time. Fuel knock can be harmful to an engine. From what I can see on the internet, knock sensors are a common replacement item on Nissans, not sure if people are replacing them because they commonly go bad, or replace them blindly, just by seeing the code. The O2 sensors in your exhaust are what indicates to your engine computer how well it's doing a job of injecting the correct amount of fuel at the right time and what adjustments it needs to do. Fault codes in electronic control systems need to be interpreted... Lots of people just see the codes and replace the sensors related to the codes. This might fix the problem sometimes, but lots of times the codes can be attributed to something else... You might change both the sensors and cure the problem, but you might dump money into replacing these parts for nothing and the problem could lie somewhere else (fuel pressure regulator, ignition components, etc...) Also it depends if these codes were "active" (still happening at the time of connecting it to the machine) or "passive" (happened at some point but not happening at the time of connecting it to the diagnostics machine). If they're active they are a lot easier to diagnose and fix than if they are inactive. I know for a fact, working in the field, inactive codes are often next to impossible to repair in decent time.