BMW 3 Series (1999-2005) Pre-Cat & Post-Cat Oxygen Sensors DIY

Replace the O2 Sensors before and after the Catalytic Converter on an E46 (1999-2005) BMW 3 Series

Works For:
1999, 2000, 2001, 2002, 2003, 2004, 2005 BMW 3 Series Models
BMW 3 Series Includes:  316, 318, 320, 323, 325, 328, 330 and M3 .

Source: Ivegotissues666 at E46Fanatics

I thought I would share my experience with replacing my post cat O2 sensors in my 323i. Overall I felt it was fairly staright forward considering there is slim to none information regarding replacing the O2 sensors in the Bentley Manual. So I decided to put together this DIY for the e46fanatics members.

Tools/Parts Needed:

2 Post Cat O2 Sensors P/N: 11-78-1-433-940
Ramps or Jack Stands
Torque Wrench (I used a 1/2″ drive with 3/8″ adapters)
O2 Sensor Socket 22mm (I used both a really short one and long one, also I forgot to take picture of them but will shortly)
Open Ended 22mm Wrench
Small Flat Head screwdriver or Utility Knife
Long Flat Head screwdriver or equivilent
Torx Bit (Sorry I forgot which Size)

Installation Time: Approximately 2-3 Hours


Put Car up on Ramps or Jack Stands (Ramps worked fine for me)


Remove the cabin microfilter shown below by removing the 3 locking tabs

Step 3:

Remove this peice that the cabing filter rest in (sorry can’t remember the name of it). It has 4 Torx bolts(I forget What Size)


Step 4:

Upon removing the torx bolts before removing this peice you must unclip the plastic trim peice that hold the + battery cable and wiring loom. Then you may remove the whole unit.

Step 5:

Now you must remove the engine fuel rail cover on your right hand side. There are 2 caps that can be popped off by using a small flat head screwdriver to expose the 2 10mm Hex bolts that need to be removed.

Step 6:

Once the fuel rail cover is removed locate the Post cat O2 sensor plugs (They are tucked in between 2 clips. You can follow the wires and notice they travel down to below the engine under the car (they are pretty long)

Step 7:

Unplug both O2 sensors by using a small falthead screwdriver or a utility knife to undo the little clips on the plugs and sockets

Step 8:

Work your way to under the car now and remove the under carriage peice that is held on by 6 phillips head screws (Note you dont have to remove your bumper well at least I dont think so anyways I removed mine because I was working on something else anyways.)


Step 9:

Once the undercarriage peice is removed now its time to locate your Post Cat O2 sensors. For me they they were pretty much directly directly to the right of my oil pan (I dont know if it the same for all e46 models)

Step 10:

Now this is where the fun begins (Note: If attempting this you may want to first see if you can remove the O2 sensors first before performing the first 8 steps just so you dont waste your time trying to remove these only to realize you can’t) The reason I say this is because these things are a royal pain to remove from them having a decent torque spec and to the fact that they are rusted and seized on as well. For the first one, the one more visible and easier to get to. I was able to use a short 3/8″ O2 sensor socket and a 1/2′ drive torque wrench w/ 3/8″ adapter to remove the first one. Now for the second one all I have to say is good luck because literally this one probably took me an hour or more jus tto remove. I had to fiddle around with the second one from using an open ended 22mm wrench, a larger O2 sensor socket and some pivoting ratchets. But alas I finally did remove it, so just keep at it. Once you break them loose initially you can untighten them the rest of the way by hand. Just be patient there is barely any clearance to get your hands, tools and sockets in that area so just keep at it.


Step 11:

Once both O2 sensors are removed you need to gently and carefully pull the wires from the sensors below the car. I did this just in case anything happend to my new O2 sensors that way I would just have back ups.

Step 12:

Now it is time to install the new O2 sensors(The installation is basically the reverse of remval except make sure to be careful that NO grease or oil or anti seize compund from the threads get onto the tip of the sensor. You can hand tighten them all the way (They will go in with barely any effort) and tighten them to the proper torque specs which are 37 ft-lb

Step 13:

Note: Please be extremely carful when do this you dont want to pull/tear a wire off of the sensor. Once the New O2 sensors are installed and torqued to spec, it is now time to reroute the O2 sensor wiring up into the engine bay. What I did was used some 16 gauge wire that I had laying around and fished it to down under the car from the engine bay. Once I got the 16 gauge wiring to the bottom where the O2 sensor were I used some painters tape (It doesnt matter which tape you use I just had some blue painters tape available at the time) and I Taped both O2 sensor wires to the 16 gauge wiring. Using this method I was able to carefully and gently pull the O2 wiring back up to the engine bay. Note: there is a clip or two between where the O2 sensors are installed at the exhaust and where they ar plug into at the engine bay. I used a long screwdiver to gently push them back onto the clips. You can only see them from underneath the car.

Step 14:

Once you have routed the wires to the engine bay it is now time to plug them back into their proper socket. The O2 sensor that was more visible and easier to remove on my car at least went to the plug socket closer to the oil filter, and the O2 sensor that was harder to remove and less visible from under the car went to the pug socket closest to where the cabin filter would be. I didnt know if they were interechangable or not so I just used the same socket housing that the old ones went to. Once you plug the Sensors back in make sure to put them back along the clips that you had uninstalled them from earlier in the previous step.

Step 15:

Once the the O2 sensors are plug back in at there connectors and tucked into their clips it is now time to put everything back together. Everything is reverse of the removal process starting with the engine fuel rail cover. Then the unit with the 4 torx screws that holds the cabin microfilter. Now reinstall the cabin filter, and undercarriage and you are all set you have now just installed your own Post-Cat O2 sensors.

I just wanted to include one last thing which is about installing your own Pre-Cat O2 sensors which are much simpler and probably would only take you 30 minutes tops. Looking into your engine bay remove your left hand side engine cover and you can easily see where the Pre-Cat O2 sensors are installed. All you will have to do is remove the old ones install the new ones and route the wires which is a lot simpler than the Post-Cat Sensors. Here are some Pictures of where they are located I had done them a couple weeks earlier and didnt feel it was necessary to make a DIY for these for they are to simple.

Anyone questions or comments or anything perhaps I missed feel free to Post or PM ME

Thanks Jared

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