So I've gotten a small bit of bad news about my build and I'm trying to understand this O2 sensor issue that's going on. I added pictures below that I received from GTM. So I knew our cars came with a pre and post cat O2 sensors. I know for sure the pre cat O2 sensors cost more than the post cat sensors. I understand they do more than just help determine AFR, but I don't have a complete understanding of why the fronts cost twice as much as the rears. In 2011 Nissan changed the front O2 sensors, and the price doubled. Looking at this damage, I may have to replace all four sensors because of an improper install before GTM got it. What makes this more infuriating is I told the shop not to mix them up, removing them before using an air tool on the headers, and to extend the wires one by one. This has been a headache I was chasing for over a year. I had one pre cat sensor replaced because I saw it reading crazy, but seeing that all four were jacked up, pre cat O2 sensor in the J tube, post cat in the free stream, jacked up wiring, melted and shorted at that.
So just a warning... If you are doing a header/ TP install yourself or at a shop, label your sensors so when you put them back in, you don't put the fronts in the J tube and the rears in the free stream. Extend one wire at a time, as there are two black wires. Don't use an air tool to remove the O2 sensors, or on the headers/cat until the sensors or off or risk damage. Removing the O2 sensors period can be difficult without damaging them, so be prepared to spend extra on replacing them. I can't stress how anal I was with the shop that did this, but in my own ignorance, I can't visually tell the difference between the two sensors. I even checked to make sure the wiring was up and away from the headers to prevent melting. I obviously still failed. Now I'm well over $1k out of pocket just to fix something that should have been right from the start.
If you are troubleshooting, there will be trouble codes, but sometimes hard to determine as some trouble codes can be cause by multiple things. I did get a P0300 and P0172. The sensors were reading very erratically from one extreme to the other. My gas mileage tanked to about 15mpg, as the ECU was dumping fuel into the engine as a safety net because it didn't know wtf to do with the AFR readings it was receiving. GTM saw the fuel trims were erratic as well. Hopefully this helps anyone else who may come across this issue
FYI, this is an excerpt of GTM's email on this problem.
""As you know, your car has a CEL P0300 and had that code when it came in. We took a peek under there and noticed that the previous shop had put the primary sensor in the J tube instead of the freestream bung. We switched them around and there was no change. It looks like the previous shop extended the wrong O2 sensor wires. In addition, the wires appear to be crossed which melted and shorted the wiring. You will need two new O2 sensors as well since they were shorted out. Because the O2 sensor wires are shorted, there is a small chance that may have damaged the ECU so we need time to diagnose your car and check the ECU. I have attached some pictures of the wiring and O2 sensors so you can see what we're talking about. Overall, we need an additional 5 hours for diagnosis, repair, re-wiring, checking the ECU, and replacing the 2 O2 sensors plus the cost of two new sensors."
