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Discussion Starter #1
I've already done some searching and reading, just looking for validation on my approach here.

2003 with 48.5k miles, car had a b2 code on it when I bought it in October at 45k miles as shown in PPI, cleared code at that time. Bought Peake a month ago, checked codes, totally clear (I danced a jig). Checked codes again two weeks ago, b2 code came up, NO SES light. Cleared it, checked tonight and now have both b2 and b3 codes, cleared codes. Still no SES light though. Car runs strong other than some slight hesitation after startup in the cold weather we've been having. Said hesitation disappears in less than a minute though.

Approach I'm thinking of:
- Replace one or both POST cat O2 sensors with Bosch units
- Check for codes
- If codes resurface, visit indy to check for rich running conditions that may be causing issue (DME)
- Possible replaced CPSs, cats (thinking magnaflow), etc as needed

Relevant maintenance done since 45k:
- Replaced spark plugs in December
- Replaced upper oil separator hoses and cleaned separators
- Oil and filter change 10w60 late December

I've been meaning to snap pics of my plugs for a "tea leaf reading", if folks think that would be helpful I can do that and post them here. Offhand the thing that stuck on the plugs to me was light corrosion on the metal of the most rearmost on the driver's side. I attributed this to a bad seal on the cover gasket....

Your thoughts and advice are appreciated, I understand this has been covered before, just want to make sure I'm following the right path.
 

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Discussion Starter #2
PS - I'm seeing mileage right around 16MPG in mostly city driving, I measure this by hand not the onboard calculation (miles/fuel added) Not sure if this helps, thought I'd throw it in there.
 

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I'm not a mechanic but I would first:

1. Check your T-stat using the "secret menu" function (easily found with a search and referenced on recent threads regarding thermostats)
2. Check for small exhaust leaks, esp. on the bungs that jut out to hold the 02 sensors

Changing the post-cat O2 sensors is inexpensive but they should be OK at < 50 K miles, and I doubt both of them would go at the same time. In fact, it is unlikely both Cats would fail at the same time...which is why I would look for something causing the car to run rich.

GL
 

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Discussion Starter #4
Thanks Kevin, I've been following this logic.

I should have noted the thermostat was replaced in November after I got a code 69 temperature implausible. My indy missed a seal the first time around causing a coolant leak, so they tore it back apart and fixed it. I did the KTEMP test today and saw steady 78 - 79C in highway driving, a little cool but nothing drastic. Air temps in the high 50s(F) here.

I emailed my indy last night about a diagnostic visit, haven't heard back yet. Hoping I can get in and out for an hours labor and have them check for rich running and eyeball some suspension bits for me.

I've got all four O2 sensors in my Amazon shopping cart, debating pulling the trigger.
 

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I was having same issues a year ago

I was getting both B2 & B3 at the same time about 1 yr ago. A number of members have had very similar issues and as they will point out, it is pretty rare for both cats to fail at exactly the same time. IF you still have your stock exhaust on (and no other mods), you may be in luck as I believe the emmissions warranty is 8yrs 80k miles. As I found out, BMW NA will NOT honor the emissions warranty if you have anything other than a Dinan exhaust on there.

In my case rather than change the cats, I replaced:

- pre-cat O2 sensors
- thermostat (I wasn't getting a code, but it was running well below normal)
- basically all the oil lines and vac lines
- spark plugs
- air filters & fuel filter

One thing I have noticed is the car is getting much better mpg on my daily commute. It was in the 11mpg range (I have a brutal commute in heavy traffic) and is now more like 13-14 around town 25-26 on the highway. The B2 & B3 codes are gone (for now), but I fear whatever rich condition I had may have gone on long enough to seriously limit the lifespan of the cats (I'm at 56k miles).
 

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Discussion Starter #6
Getbent,

I'm told CA warranty is 8/80k, federal is 5/50k. A previous owner installed Magnaflow mufflers, so I'm shot on that front with BMW. The welds looked solid if sloppy; sounds like I should check for leaks, though I'm not sure how to do this other than getting under the car with it running which seems somewhat dicey...

Thanks for your list, notice you didn't replace the post cats, was this due to cost or just reflecting the collective wisdom around here that the post cats don't usually go?

Of these:
- pre-cat O2 sensors (on order)
- thermostat (replaced in October)
- basically all the oil lines and vac lines (just upper oil seperators, prolly wise to do the rest)
- spark plugs (replaced in December)
- air filters & fuel filter (have fuel filter, will do this month, air filters on order, though they look fine)
 

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I should have noted the thermostat was replaced in November after I got a code 69 temperature implausible. My indy missed a seal the first time around causing a coolant leak, so they tore it back apart and fixed it. I did the KTEMP test today and saw steady 78 - 79C in highway driving, a little cool but nothing drastic. Air temps in the high 50s(F) here.
Do you recall which seal was missed? Is it the $30 one that sits atop the tstat housing or simply one of the myriad o-rings/gaskets? I recently had my tstat replaced, but still running 75C cruising.

RE: b2/b3, have you checked your fuel trim to determine if it's running rich? That will have an effect on throwing those codes. Cheaper to check FT, change O2s then cats, that's the route I'd take.
 

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Discussion Starter #8
Do you recall which seal was missed? Is it the $30 one that sits atop the tstat housing or simply one of the myriad o-rings/gaskets? I recently had my tstat replaced, but still running 75C cruising.

RE: b2/b3, have you checked your fuel trim to determine if it's running rich? That will have an effect on throwing those codes. Cheaper to check FT, change O2s then cats, that's the route I'd take.
They told me a seal or o-ring had been crushed in re-assembly, I don't know which one. The leak was not obviously coming from the thermostat, I think it was dripping out through the valley of the V in the engine and then onto the ground. It was a pretty quick leak though, went through a fair amount of coolant. I'll check my paperwork later tonight or tomorrow and see if they noted the part #.

Can I check the fuel trim myself via OBC or some other method? I did a maf test in late October and corrected to 139 I believe. I emailed my indy to see about having them hook it up to a GT1 or what not to get more information on a possible rich condition. I'll do the pre-cat O2s regardless, seems like good preventative maintenance from what I read here.
 

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They told me a seal or o-ring had been crushed in re-assembly, I don't know which one. The leak was not obviously coming from the thermostat, I think it was dripping out through the valley of the V in the engine and then onto the ground. It was a pretty quick leak though, went through a fair amount of coolant. I'll check my paperwork later tonight or tomorrow and see if they noted the part #.
That sounds like one of the O-ring snapped, in my case, there was no leakage afaik, but it fails to maintain temp, still waiting on shop's diagnosis.

Can I check the fuel trim myself via OBC or some other method? I did a maf test in late October and corrected to 139 I believe. I emailed my indy to see about having them hook it up to a GT1 or what not to get more information on a possible rich condition. I'll do the pre-cat O2s regardless, seems like good preventative maintenance from what I read here.
I believe the FT can be read by DIS/GT1, not OBC. There're 2 sets of values, long term FT (additive) and short term FT (multiplicative), I may have them reversed. The pre-cat O2s are used as parameters for engine management, I think post-cat O2s are responsible for detecting b2/b3 codes, otherwise how would it know the cats are bad without the exhaust gas having traveled through the cats yet? Just thinking out loud here.
 

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RE Post-cat O2s

I didn't replace them based on some threads I had read at the time, but it certainly wouldn't hurt anything either. I definately had a couple vac leaks because some of the lines basically crumbled when I was changing them out. That plus the bum thermo were likely causing my car to run rich. Its been OK (knock on wood) for a few months so I'm in the wait and see mode. As you'll see on the board these cars can run rich for a number of reasons and in some instances burn oil (less likely w/ your 2003). Hard to tell if the M5s eat cats more frequently than any other models but there are a lot of threads on the board related to the subject.

If it does turn out your cats are dead you might still want to try and see if you can get BMW NA to replace. Worth a shot at the dealer in any event. You could also just buy or borrow some orginal cans for the service visit.
 

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Seems to me these codes are related to even combustion. Anything that throws that off could be suspect. First thing I would look at would be fuel pressure and volume. When was the last fuel filter change? Do you ever use injector cleaner? If the fuel pressure is down or if you have one or more dirty injectors there may be an excess of hydro carbons (unburned fuel) for the first O2 sensor to grasp. So when the second o2 sensor sees what is left over it will not be what the computer has in the data base and will report that the cats are not working correctly. Simplified explanation, but basically accurate. If the fuel is not properly atomized you will get unburned fuel at the cats. First thing to look at for these codes would be complete combustion, which would be air also but I think you have covered the air from your description.

The way your codes appeared I might suspect a fuel filter problem because b2 appeared then later b3. b2 is the side you will see low pressure hit first then as the filter gets more plugged it would affect the whole fuel rail. The old e34 way of delivering fuel to and from the rail was much better.

My b3 appeared then a month later I got a vanos code when I corrected the vanos the b3 was gone also. I was not getting perfect combustion.
 

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Discussion Starter #12
@GetBent - Austin BMW doesn't strike me as charitable place, but this may be worth pursuing
@Sailor - good thoughts, car has original fuel filter, I don't use a cleaner. I have replacement filter and washer on hand

Maintenance this weekend:
- Replace pre-cat O2 sensors
- Replace fuel filter
- Replace burnt brake bulb (my kinda fix, cheap & easy)

Indy appointment on Monday:
- Check for rich fuel running conditions
- Check fuel pressure stats
- Check for exhaust leaks
- Check vacuum lines

Thanks folks!
 

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Discussion Starter #13
Noticed a rattle from the rear right of the car leaving intersections and lights. The rear right muffler seems loose, I'll take a closer look when I'm under the car this weekend to do the O2 sensors and fuel filter. If the weld has failed somewhere this could explain back pressure and exhaust issues contributing to the codes....
 

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Have you done a fuel flow test and run a complete GT1 diagnostic? Very interested to know what AFRs you are running and fuel trims....
 

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Discussion Starter #16
@E55 - the RSBs are good, replaced with new BMW units before I bought it, can still see the stickers. The magnaflows only use one of the muffler mounts in the back, the passenger's side is a bit loose, pretty sure this is the source of the occasional rattle.

@ard - appointment with my Indy tomorrow to hook up the GT1 and check things out. I have AFR and fuel trims marked down, anything else I should ask them to look at with the GT1?

Replaced the original fuel filter today, I didn't see any of the black good others have described here. I did notice that the original unit was noticeably heavier than the Hengst replacement, so it was doing something right. :) I also replaced the two pre-cat O2 sensors, the old units were black, but after 8 years and almost 50k miles not sure that's surprising. Those suckers were on TIGHT, took some mallet action and a big *** wrench to pop them loose.

Wet outside today in Austin so hard to really get on it, but the car performs well and seems to stutter a little less in the cold start scenario.
 
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