Time required: ~30 minutes or less
Skills required: wire stripping, fine motor skills
Tools needed: 8mm socket for the underbody panels, 22mm wrench (or 7/8") for the O2 sensor, stripper for 18 awg wire, wire cutters, maybe some scissors or a sharp knife also.
Parts needed: Bosch Universal O2 sensor, part #15733 (already has anti-seize on the threads), 1x ziptie
Okay, so like many out there I got an O2 sensor error code after I installed my catless downpipes. How did this happen? I'm not sure, as I was pretty careful not to twist cords or bang around my sensors when swapping them out from the OEM pipes to the new ones. Anyways, the code I got was "2C9E Lambda probe heating behind catalytic converter," which is bank 1 after the cats, aka front turbo downstream. For those who don't know what I'm talking about, reference the following diagram from realoem:
Bank 1 = cylinders 1-3 = Front turbo = Driver's side DP (LHD) = Green o-ring connector
Bank 2 = cylinders 4-6 = Rear turbo = Passenger side DP (again, LHD) = Yellow o-ring connector
The N54 has four O2 sensors, two per downpipe, one before the cat and one after. The ones before the cat are wideband (5-wire) and should only be replaced without cutting wires or soldering - OEM is the only way to go I guess. The ones after the cat are universal (4-wire) so no big deal. I had to replace #4, as per the realoem diagram. I checked out the prices for the OEM replacement and they were ~$179 each
So I decided to go for the Bosch Universal, part 15733. $67.99 at Autozone, before military discount
You will have to remove the center underbody panel using the 8mm socket. You will not need to remove the main underbody panel in order to access the rear O2 sensors. Remove the old O2 sensor using the 22mm wrench. Lefty-loosy, righty-tighty. If you disconnect the sensor first and straighten out the wiring it will be easier to unscrew the O2 sensor from the downpipe.
Here is the old and the new side by side. Notice you will have to transfer the shielding from the old to the new. You want to cut the wire on the new sensor so that it matches the length of the cut on the old sensor (does that make sense?) and you will reuse the original connector.
Cut the ziptie on the old wiring that keeps the heat shield in place in order to slide it off. DON'T FORGET TO TRANSFER THE HEAT SHIELD TO THE NEW WIRING!
There is a gap in pictures here, but what's missing isn't hard to visualize.
I cut the wires and strip 3/8" off each end. Slide the rubber sheaths off the wiring enough so you can cut 1.5" off to accomodate the connector box. You will do this to both the sensor wiring and the connector wiring (sorry, no pics). Clarification: you should have two pieces of 1.5" sheathing to throw away now.
The black cap goes on the connector side and you slide the yellow grommets onto each wire so it will look like this (make sure you orient the yellow grommets so the ribbing will be on the expose wire side, check out the image on the box in the first picture if you're confused). The grommets make the connections waterproof.
Unscrew the ends off of the grey connectors and slip them onto the wires one by one (image is the o2 sensor side with the black box). MAKE SURE THAT THE WIRES ON THE BLACK CAP MATCH UP WITH THE WIRES ON THE BLACK BOX. One grey, one black, two white.
The wires are pressed into these metal cones connections inside the grey connectors when you screw the ends on. After you tighten the ends, give them a little tug to make sure the wires are secure.
Finished connections. Make sure your wires match up!
Slide the grey connectors inside the box and connect the cap.
Below is an image of the two rear O2 connectors. Notice the yellow and green o-rings. Also notice that the two connectors are different so there is no way you can mix them up - foolproof! (Just an FYI)
I tucked the box behind the bracket so it's not dangling - you can ziptie it if you want.
Final notes: Don't forget to transfer the heat shield to the new wiring before you start putting on the black box and the yellow grommets. The rubber sheathing will need to be scrunched up on the O2 sensor side in order to work with the wires - this will be annoying as you try and slide the grommets on and connect the grey connectors. After I buttoned everything back up, I cranked up the car and drove for a bit - no codes! I ran through all the resets with my BT tool, though I don't think it's necessary.
Pat yourself on the back for saving over $100.