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

440K views 134 replies 75 participants last post by  Herminshs 
#1 ·
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)



 
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#64 ·
Here are some pics from an overhaul a couple months ago.

In order to do this job, you have to take off the reinforcement plate (mine had one), then it is easier to access these sensors as well as to route the wiring. Dropping the exhaust is not necessary in contrasts to what the TIS says as this is proof.

Bank 1 has this stupid dummy bracket, observe in sdbimmer's diagram in above posts. You can see it in my first few pictures. I used a dremmel bit made for metal and sawed off this sucker because it was preventing me from getting my socket and extension in. A word of caution that when you cut this thing off, sparks will fly. Also let your dremmel or alike rest after trying to cut. If you don't you can burn out the motor. Ask me how I know. :facepalm:

You'll also see that I painted it. I mainly painted it so that the rust would not eat through the pipe. Granted that there is already rust on the surface, but overtime, I think a freshly cut metal may accelerate the rust, so I used high temp paint. High temp primer first then the paint, two coats each.

The 4th picture, has the tool setup on how to remove the bank1 sensor, note in the picture that you have to saw off the bracket first. Third and sixth picture show how I used a small adjustable that was thin enough and could open wide enough to slip between the pipes. From there I used a three pound sledge and gave it a wack to loosen the seize. Be sure to wack the correct way otherwise your tightening it. Just look at the threads on a new o2 sensor to see which way one should wack it.

7th and 8th picture shows the routing of the wiring. Once you remove the reinforcement plate or equivalent, you can easily maneuver the wiring to their mounts. I would drop the new sensors in from the top. I tied additional strings to them in order to get them down. Be sure to route in the same manner as the originals, so take lots of pictures.

The last shows one of the post cat o2's with over 97k on it. I did not have any issues as far as why I replaced it but simply for the fact that these typically only last 100k and I did them as part of an overhaul with sensors and just general maintenance. I replaced my pre-cats too along with a boat load of other stuff.

After 8 or 9 months of overhaul, I was finally able to enjoy getting her back on the road. Initially lots of DTCs were stored once I plugged the battery back in. No SES light. I just cleared the codes and after 350 miles so far this week, no codes, no problems, no leaks, and no issues. :woot: :thumbsup:
 

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#76 ·
I had SES light on with 165k and replaced all three 02 sensors with botsh and light is gone. I was not able to chanege the 2nd pre cat and I gave up becouse the space is really tight there almost impossible to apply force and I do not want to cut the brackets to remove it.
So my question is it ok to leave this sucker on as it not giving any codes?
 
#82 ·
quote...
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.
Unquote..

My 02 330ci has 127k miles and I did not know if the sensors were orig. or not but just to be on the the safe side I decided to change the pre cats and here are some observations.


- I bought mine through Amazon, they had the exact same markings as the orig. i pulled out except the lack of "Germany" marking he new one.. price shipped was about $95 for 2 sensors

- for some reason, the OEM O2 sensor tool(spark plug socket looking thing) i bought DID NOT fit(too small) I ended up renting the tool set.

- If you are doing the pre cats ONLY, there is no need to take out the cabin filter assembly. the fuel rail cover comes off pretty easily. just like the OP says.

- it's easier to use the non-spark plug type tool to get the sensor loose and undo by hand (vice versa for installation: screw on the sensor by hand and
tighten snug with tool). the spark plug type socket makes you fold the top part of the wire which cannot be good

- the sensors come off pretty much hassle free. (the last set of o2 sensor i did was on a 97 Nissan Maxima and that was a good 2 hr job involving a torch penetrating fluid.)

- whole process took 40 mins including 20 mins to rent the tool...

car drives better in 1st gear and i don't notice knock/ping when i'm trying to accelerate from low rpm (lets say 4th gear 1500 rpm)
 
#87 · (Edited)
Just did the post cat sensors 2 days ago. I will NEVER do that again, unless i had the right tools. While under there now i understand that a 22mm open end wrench would've proved invaluable, especially if its thin. That or a wobbly socket so you can have your ratchet in there with a better angle. Also an adjustable extension socket would do too. If you have a problem breaking them loose find something to use as leverage. I didnt have to muscle anything. I used the long pole attached to my quick pump low jack. That bar was a life saver. After attacking one of the 6 (?) bolt holding on the shield covering oil pan that wouldnt come loose, that bar did it without event a grunt.
 

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#88 · (Edited)
Some thoughts on O2 sensor changes

Hi, all. I just did the four O2 sensors on the wife's 325i (MY 2002, 2/02 build, automatic). The DIY is very useful, but here are some comments.

The 7/8" sensor socket would not fit over the installed pre-sensors. Interesting because the socket would fit over the new sensors. Instead I used a 7/8" crowfoot and the sensors were replaced with out any problems. I did use a little Liquid Wrench on the old sensors.

The underside of my car is different from that in the photos. There is a plastic pan under the engine and an aluminum pan under the tranny. The aluminum pan is held in by eight bolts that require a 16mm (5/8") socket. Once this pan is removed access to the post-sensors is very good. I was able to R&R them with a 22mm box-end. Again, I used Liquid Wrench on the old sensors. I used a straightened coat hanger taped to the new sensor plug to pull the cable up from behind the tranny.

BTW, on my car for both the pre- and post- sensors the front plug led to the front sensor and the rear plug to the rear sensor. Thus, it was very easy to ensure that the right sensor was connected to the right plug.

Boy, did I save a ton of money doing it myself.

Thank you for the DIY.

Dan
 
#89 · (Edited)
Which o2 sensor to change? Have codes

I just got a CEL on my 2003 325iT; ran a scan with engine cold and engine warm, with BMW scanner 1.4. Cleared the errors but some came back right away. I can't figure out which bank or sensor is faulty?

Codes:
296b [ 10603 ] lambda sensors after cat switched (code that keeps coming back many times)

Got this code 1 time as car was warming:
28F3 [10483] Lamdasensor bank2: trimm control

29A0 [10656 ] O2-sensor behind KAT Bank 1:signal
(old original code that didn't come back when I scanned for a while)

BMW price for one post-cat sensor here is $290 + tax, what part # is of equal quality from Napa or national car parts chain?

Is bank 1 the easier one to change? Pls advise I can buy the part now and try the diy.
:thanks:
 
#91 · (Edited)
I just got a CEL on my 2003 325iT; ran a scan with engine cold and engine warm, with BMW scanner 1.4. Cleared the errors but some came back right away. I can't figure out which bank or sensor is faulty?

Codes:
296b [ 10603 ] lambda sensors after cat switched (code that keeps coming back many times)

Got this code 1 time as car was warming:
28F3 [10483] Lamdasensor bank2: trimm control

29A0 [10656 ] O2-sensor behind KAT Bank 1:signal
(old original code that didn't come back when I scanned for a while)

BMW price for one post-cat sensor here is $290 + tax, what part # is of equal quality from Napa or national car parts chain?

Is bank 1 the easier one to change? Pls advise I can buy the part now and try the diy.
:thanks:
As far as which one is easier to change, none of the post cats are easy, and you might as well change them both out. These things are always changed either in pairs, or all at once. They could be originals and need to be changed around 100,000 miles. One of mine went out around 130,000 so that's when I changed them all out. Regarding replacements, be careful of sellers who tell you that they are bosch but in fact are Chinese fakes. I bought mine on ebay. Wherever you buy it at, make sure you ask them if they will be in original bosch boxes and make sure the boxes are there when you receive them. If not they're counterfeits. The pre cat o2s I bought came in a white box with a Sticker saying bosch. Inside they were in a clear bag with some sort of part# i didn't recognize. It was from some seller named abc...Something. Do not buy from these lying a$$ frauds. I called them up complaining that they sold me Chinese fakes. The guy on the phone (who I could easily tell was chinese) denies my claim but i eventually got them to pay return shipping and refund my money.
 
#90 · (Edited)
I bought all 4 of my o2s for around $210. I have a 330ci. They're bosch o2s, same ones used in our cars, just not the "genuine" bmw let me rip you off ones. I remembered that one part # is 13477.. The other was like 105......don't remember the rest. Google it with the word bosch in front and o2 afterwards. You'll come up with something
 
#93 ·
The one from auto zone comes with 3 different type of o2 sockets or at least the one that I rented was. I ended up using the open top one. Looked like someone. Snapped off the end of a wrench and cut a slit into it.
 
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