I have 2003 G35, and there no issue with the knock sensor yet, there few video on youtube show how to replace wire harness, good luck

 

The knock sensor code is almost always the sub harness, some folks have managed to change it out without disassembling the intake but the normal process is to remove the upper/lower plenums and the collecter intake at the bottom. Then swap the sub-harness (since you have it all apart you should just replace the entire sensor as well since it's a $20 part that probably has 100k+ miles on it). Then reassemble, it's not HARD to replace but it's a lot of little bolts and you need to follow the FSM for proper torque and pattern. If you aren't the type to actually follow the manual or torque fasteners I would opt for the other method because you will probably just screw something up.

The lean bank 1 code however could be a number of different things ranging from failed O2 sensor to failed fuel dampers to bad spark plugs to intake leaks, etc. Expect quite a bit of troubleshooting to find the actual issue.

I would consider a vehicle like that to be sold as a "mechanics special" and have a cost reduced by -30% due to the amount of labor needed to fix the problem.

If you know how to troubleshoot engine problems like this and will do the work yourself it's not a problem, if you don't know how or will be paying a shop to do the work then just walk away now because it will easily be $1k to fix those problems.

 

Don't rule out the knock sensor until it's tested. Some models have more trouble than others with the knock sensor itself. Example is the G20, The 2000 and up almost never have a knock sensor failure but the 1999 down models fail all of the time. I don't know if it's how the knock sensors are driven from the ECU or if it's just certain types that fail more often but they do fail. The G35 sensors may be more reliable since they are slightly newer. Please report back what you find, I would be interested in hearing what the failure is/was. If it is the harness I would like to know what failed. Does the crimped pins come apart over time? Oil/contamination get into the connectors?

 

The ones that have a sub harness problem usually have brittle and cracked insulation on the wires that eventually rot away or end up making contact with each other.