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Hi all,

I got some great info from previous posts on this topic, but I haven't been able to crack my problem yet. I'm hoping you all can help.

2002 M3 w/ 68K miles. CEL intermittently comes on, throwing the P0420 and/or P0430 codes. Also the "Rich to Lean sensor threshold voltage" is failing its test.

Engine temp stays within the 180-187F range. Attached is O2 sensor data for a cold start, as well as a 10-min drive cycle during which the CEL came back on.

To me it looks like the O2 sensors are working fine and the cats are doing okay at idle. However at highway cruise, the post-cat O2 sensors are drifting a lot. Does this indicate failing cats? Should I be suspicious of anything else?

Thanks in advance!
 

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Hm, at 68k miles I would not think of either part failing, but I guess anything can break over time. You're past the EPA warranty on the cat, too.

It is hard to tell from your graphs so I'll have to ask: Is it just one bank being weird? If so, swap O2s and see if the problem follows the switch. If it does then it's the O2. If not, it's the cat.
 

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Discussion Starter #4
Both banks seem to be behaving similarly according to the graph. This is consistent with the codes: often it throws P0430 and the other bank (P0420) is "pending".

I'd also be surprised if cats failed at this mileage, but I suppose it's 15 years old already...
 

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When just seeing a few graphs and not being able to look at the big picture, it appears the catalytic converters are working well at idle by cannot store enough Oxygen while cruising assuming there are not other problems.

Car has low mileage, so there may be carbon build up on the converter surface unless they have been contaminated by oil if the car burns/uses a lot of oil.

I might be inclined to take a 1 hour road trip with the transmission down 1 gear to see if you could burn off any contaminates in the converters. 3500-3800 RPM for 1 hour might make a difference?
 

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@jfoj, that's a good idea. Has anyone had success using any of the fuel additives for cleaning cats?

I reset the codes and was able to pass the CA smog test this morning, so maybe they're not totally shot yet and there's a chance of recovering some performance...
 

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@Thunderstruck Did you drive it around enough to reset the "readiness"? Or were you able to go straight there to test it?
 
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