DIY: Post-Cat O2 Sensors + Pre-Cat O2 Sensors

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

Step1:

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

Step2:

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

at what milage should the post-cat be changed?? I change my pre-cat at 50k...great diy by the why

Same things for Post O2's 100k

And Thanks

I just replaced my rears yesterday. This guide was helpful but I would like to add 1 thing.

When pulling out the old rear ones, I tied a string to the end on the top side, so when I needed to route the new sensors wire back to the top, I just tied it to the string and pulled.

excellent diy write up. what a PIA though! btw, i found the post cat o2 (lambda) sensors on realoem.com but cant find the pre-cat senors, ne1 have the part numbers for those?
Post cat:

just found the write up for the pre-cats, thought i would cross-link: http://forum.e46fanatics.com/showthr...=o2+sensor+diy

nice DIY
I need to do this soon

If I disconnect my pre-cat 02 sensors will this allow me to assess whether my power problems are due to a bad Cat? i.e. if my power problems are solved its the bad reading from the o2 sensors causing a poor air/fuel mix. Will disconnecting the pre-cat 02 sensors allow this?

bumpin to remind myself to change these
I think i'll do the precats first then post when im not as lazy

Great write up

Great write up, very useful but one thing missing.

Error codes refer to bank 1 or bank 2. As I understand it, bank 1 is cylinders 1-3, bank 2 is cylinders 4-6.

Although when you get into the job you can probably work it out (am I right assuming cyl 1 is at the front and cyl 6 at the back ?), it would be really useful if the sensors and connectors in the pictures had been labelled as to which was the bank 1 or bank 2 post-cat sensor and plug. If I get a bank 1 error (P0420 - cat bank 1 below efficiency actually, but the car's passed an emission test in the MOT so I suspect post-cat sensor at fault, not cat) I'd like to know which sensor to look at (so I know in advance whether it's the "easy" one or the "hard" one I'm looking at changing).

Can anyone help with that info ?

Vexed !!!

Cheapest price I can find for OEM (post-cat sensors) Bosch part number 0258005109 in UK is: Bosch Direct Fit - (wires 4, cable length 990mm) £71.68 inc VAT & UK deliv. (= $139.55 USD !!!)

That's $80 more, for the same OEM Bosch part that I've seen advertised in the US for $58-$60 !!!

http://www.lambdasensor.com/main/bos...tem=0258005109

http://www.rockauto.com/catalog/rafr...1433864k279826

http://www.autohausaz.com/search/pro...xygen%20Sensor

Can anyone recommend a supplier who will supply at similar prices and ship to UK at a reasonable price ? I'm gonna have to check around the web further too I think.

Originally Posted by tony325ci_51

Great write up, very useful but one thing missing.

Error codes refer to bank 1 or bank 2. As I understand it, bank 1 is cylinders 1-3, bank 2 is cylinders 4-6.

Although when you get into the job you can probably work it out (am I right assuming cyl 1 is at the front and cyl 6 at the back ?), it would be really useful if the sensors and connectors in the pictures had been labelled as to which was the bank 1 or bank 2 post-cat sensor and plug. If I get a bank 1 error (P0420 - cat bank 1 below efficiency actually, but the car's passed an emission test in the MOT so I suspect post-cat sensor at fault, not cat) I'd like to know which sensor to look at (so I know in advance whether it's the "easy" one or the "hard" one I'm looking at changing).

Can anyone help with that info ?

Hey Tony, I have the exact same error on my 03 M3 saying htat efficiency for bank 1 is below threshold. Did you fix your problem and did the light go off? I was just wondering if you could help me out because I'd like to do it myself. Was it a post-cat sensor and did you just have to replace one? Any help would be appreciated. Thanks.

Originally Posted by hoseb'sm3

Hey Tony, I have the exact same error on my 03 M3 saying htat efficiency for bank 1 is below threshold. Did you fix your problem and did the light go off? I was just wondering if you could help me out because I'd like to do it myself. Was it a post-cat sensor and did you just have to replace one? Any help would be appreciated. Thanks.

Hi, as I mentioned, the car's passed an emission test in the MOT so I'm just guessing it's the post-cat sensor at fault, not the cat.

I only very rarely get the error (P0420 - cat bank 1 below efficiency), maybe once every month or two, and almost always on the same stretch of road where the engine is under constant load up an incline on a long stretch at a constant 55-60mph speed for several minutes (probably occurred 6 times on this road over the last couple of years, only once ever anywhere else, even when using cruise control at similar speeds elsewhere), and I find that if I vary the throttle occasionally, even on this road it doesn't happen.

As it's so rare and intermittent I haven't bothered trying to find a fix, I just hook up the laptop, clear the code each time and expect that maybe in 6 weeks time on the same stretch of road it might pop up again. As long as the car passes it's annual MOT emission test It's not really worrying me.

Here's how I diagnose and clear the code btw:
http://forum.e46fanatics.com/showthread.php?t=576850

Originally Posted by tony325ci_51

Hi, as I mentioned, the car's passed an emission test in the MOT so I'm just guessing it's the post-cat sensor at fault, not the cat.

I only very rarely get the error (P0420 - cat bank 1 below efficiency), maybe once every month or two, and almost always on the same stretch of road where the engine is under constant load up an incline on a long stretch at a constant 55-60mph speed for several minutes (probably occurred 6 times on this road over the last couple of years, only once ever anywhere else, even when using cruise control at similar speeds elsewhere), and I find that if I vary the throttle occasionally, even on this road it doesn't happen.

As it's so rare and intermittent I haven't bothered trying to find a fix, I just hook up the laptop, clear the code each time and expect that maybe in 6 weeks time on the same stretch of road it might pop up again. As long as the car passes it's annual MOT emission test It's not really worrying me.

Here's how I diagnose and clear the code btw:
http://forum.e46fanatics.com/showthread.php?t=576850

Wow, thanks man! that helps a lot. The engine light came on once before on the same stretch on the same highway. I was just cruising at about the same speed and it seemed to come on for no reason. I took it into BMW to have them do the valve adjustment figuring that was the problem anyways and it was cleared. Just recently I was driving down the same road at the same speed and sure enough that light comes on again. BMW had told me the problem was something with emissions at first but now what you tell me makes perfect sense. So if I just leave it it won't hurt anything?

Originally Posted by hoseb'sm3

BMW had told me the problem was something with emissions at first but now what you tell me makes perfect sense. So if I just leave it it won't hurt anything?

Glad to be of help. Yeah something to do with emissions sure, but what ?
Like I say I'm only guessing but I figure it's the post-cat sensor performance being degraded.

Maybe the cat is failing, but if the car's still passing emissions tests then the drop in the cat's efficiency can't be very much, certainly not enough for me to worry about or spend $100's to replace.

While it's so intermittent it's not something I'd worry about, it doesn't seem to be affecting performance or fuel consumption in any way. What I will say though is do reset the fault code light, otherwise if something else happened to cause a fault code and your light was already on you won't know.