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

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.

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.

Originally Posted by tony325ci_51

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.

Hey just to let you know for future reference, also to anyone that has the same problem. That "service engine soon" light for that code that came up cleared itself after a couple of weeks. I was going to buy the cable to clear the code and light but didn't end up needing to. It's probably better to have it in case you have the same problem but it can clear itself with time.

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.d...m=330234840067

.

Originally Posted by hoseb'sm3

Hey just to let you know for future reference, also to anyone that has the same problem. That "service engine soon" light for that code that came up cleared itself after a couple of weeks. I was going to buy the cable to clear the code and light but didn't end up needing to. It's probably better to have it in case you have the same problem but it can clear itself with time.

The engine light will clear after 3 error-free drive cycles. The error code itself will be stored in memory, and cleared after 40 error-free drive cycles, or in the case of a catalyst-damaging fault it will remain in the memory until you're done 80 error-free drive cycles. You can read all about how this works here, download the document, it's on P.22:

http://www.ebooksquad.com/2007/11/08...54-engine.html

BTW Guys who have done post-cat sensor replacements, just how do you get the wire out and back in again to that top clip? I can't reach it or even get anywhere near it.
(This is probably why Haynes say you have to drop the whole exhaust to fit new post-cat sensors, but I'm sure there must be a way without doing that).

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).

i had my 323 coded and they came up with a p1189 02 sensor bank 2 control deviation, p1188 02 sensor bank 1 deviation and p0173 02 sensor bank 1 control limit. does anyone know what these mean if so it would help alot
thanks chantz

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.

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

I just did my pre-cat sensors and I'm doing the Post-cat next. Btw, I bought them from Autozone. They are Bosch and cost $72 each for the pre, and $58 each for the post (PY upto 3/2003). Not bad at all. They also lend me the sockets for free.

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

Hey guys, just so you know we sell these sensors for $59.85 and $69.99.

http://www.fcpgroton.com/category-ex...404/by_year/51

If you buy both and your order goes over $99, enter 'SEPTPROMO' to get 10% off.

Underneath, can you tell me which postcat sensor is for bank 1-3? I only want to switch that one. thanks and great write up. This will save me hundreds of dollars!

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.

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-

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?

Could someone please tell me If this diagram also shows a post-cat sensor, as I have brought the sensor with this part number and am hoping its the same thing