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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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#29 · (Edited)
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 ?
 
#36 · (Edited)
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 ?
Anyone?
Which POST CAT sensor is for cyl #4-6??
Summer must be here, we had a warm day on Sunday so I got outside under the car and did an oil change.
While I was under there I thought I'd trace the exhaust pipes, finally take some pics and answer this question.

Both post-cat sensors are easy to reach but the Bank 2 sensor (cyl 4-6) will be more fiddly to undo, as the 22mm "sensor socket" (with a slot in it, picture attached) will have to be turned and repositioned just like with a normal spanner. Due to the wires you can't just undo it with a rachet. An open-ended 22mm spanner will be fine for Bank 1 (cyl 1-3) sensor (You couldn't use a "sensor socket" on this, there isn't enough clearance to get a rachet on it.).

(You won't have the advantage of being able to see the view in the 2nd picture. This was taken by holding a small camera in the gap up above the exhaust).

Photos include details of post-cat sensors, and the tools required to replace each:




 

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#30 ·
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 !!! :mad:

http://www.lambdasensor.com/main/bosch/price.pl?item=0258005109

http://www.rockauto.com/catalog/raframecatalog.php?carcode=1433864&parttype=5132&a=UKc1433864k279826

http://www.autohausaz.com/search/pr...cid=19@Emission System&gid=5132@Oxygen Sensor

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.
 
#32 ·
guys, my car is giving faults:

-AfterCat oxygen sensor short to B+, Cyl #4-6
Peake Research code: 15 9F...I keep getting this code

-AfterCat oxygen sensor disconnection, Cyl #4-6
Peake Research code: 15 D8...I only had this code once



So I am only going to replace the one post-cat O2 sensor, not both.
Is that OK? Or is there some reason I am unaware of that I should do both?

Also Im not sure which sensor is for cyl #1-3 and cyl #4-6. Does anyone know?

Any other advice is appreciated
Thanks for the help!!!!
 
#44 ·
My 323 has 112,00 Just did my post-cats. I have not done my fronts. My mechanics says that most O2 sensors fail around 100K but some last much longer so it is best to wait for the SES light and codes to tell you they are bad and then just replace the bad ones. The pics on this thread are great. I used a short O2 crowsfoot socket and a 3/8 ratchet with a joint (pic) to get the one that sticks straight up. I used the deep O2 socket (pic) and a 3/8 ratchet for the one at an angle. To thread them through, I just cleaned off the exhaust end of the old sensor and taped the plug end of the new one to it. Then I had someone carefully pull the old one up from on top. I also found that I only needed to jack up the front driver's side of the car to get good clearance.
 

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#46 · (Edited)
My car's recently passed another MOT / emissions test so I know emissions are ok.
I'm still getting error codes occasionally (P0420 - cat bank 1 below efficiency). I still suspect bank 1 post-cat sensor.

I've just ordered a cheap non-oem 3rd party O2/lambda sensor from ebay. Quite a bargain at £33.70 inc postage.

Equivalent part numbers:
BMW 11 78 1 433 940 ( 11781433940 )
Bosch 0258 005 109

We'll see what happens when it arrives and I fit it.

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&rd=1&item=330234840067

.
 
#48 ·
Just fyi reading step 10 (great diy btw!): I remember reading in some tool magazine somewhere that you shouldn't untighten things with a torque wrench, use a regular one for unscrewing... torque wrench should only be used to tighten stuff or it'll screw up its calibration (dunno how true this is).
 
#50 · (Edited)
I just changed my precat O2. 104K. Keep getting some codes for misfire and bank 2 too lean...etc. Not too bad. Not enough clearance except for the deep socket which could have been little longer. I bought a set of 3 sockets from amazon around $17, and the O2 sensors from bavauto for around $55 each. They were Bosch even though the web site does not say what brand. Not sure what brand ECS Tuning was selling, but it was twice as much more.

I just cleared the codes after changing them, so keep my fingers crossed that was it. If not, will have to cleaned the MAF sensor again, and change out the air intake hose for possible vacuum leak.

Great pictures. Very helpful if I have to change out the postcat sensors.
 
#51 ·
dude i got a 99 323i with 85000 miles and im getting three codes. the codes are p 1250 (manufac. air fuel metering malfunction) p1188, and p1189 which are the same as the p1250. noone ive asked has been able to give me an answer as to what i should do? i dont have time to take my car to the dealer ( i work 6 days a week from 8-5 and my only day off is sunday) could it be a bad maf? should i replace the 02 sensors? any help would be great i figured id ask since you have have a 323 and know about the car..thanks
 
#53 ·
Great write-up I just finished the replacement.

Does anybody know the post cat wiring set up. Does it matter which post cat is plugged into which connector. I marked the old o2 sensors on the car, but lost track of which was which when installing the new once. I did not mark the top wiring from the car. It's a wonder that I preformed this DIY with ease but yet wasn't smart enough to label the top connector wires for the pos cats.

The car runs fine with no error codes, I looked at "realoem" at diagram but did not help.
Thanks Paul
 
#54 ·
#57 ·
Wow! This was not easy to do. If you are going to attempt this, do yourself a favor and go to Autozone and rent two things. The first is an oxygen sensor kit. It comes with two crows foot 02 sockets and one regular 02 socket. The second is an oxygen sensor wrench. It looks like an oxygen sensor socket that is attached to a curved breaker bar. I used the wrench to remove the first 02 sensor and the 3/8 drive crows foot socket with a small socket extension + socket wrench to take off the second sensor. It was difficult. I finally got them off, but it got too dark for me to install the new ones. Will do that tomorrow.
 
#58 ·
Although good, this DIY is primarily for the PostCat sensors.

My concern is the PreCat sensors......
.....How easy is it to do the PreCat sensors?
Are they fully accessable from the top of the engine?
Any tips or tricks?
Anyone have detailed pics of the PreCat sensors installation?

Thanks in advance-
 
#59 · (Edited)
After a bit more searching, this thread seems to be more appropriate.

Which connector is for which post cat oxygen sensor? My tape flag disappeared and I don't remember which is which.
http://forum.e46fanatics.com/showthread.php?t=621322

Thanks

EDIT: I sort of remember the connectors being in the obvious position of the bank 1 connector in the front (over bank 1, cyl 1-3) and the bank 2 connector in the back. Anyone remember which is which?
 
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