Catalyst efficiency in banks 1-4 and 5-8 - Page 2 - The Unofficial BMW M5 Messageboard (m5board.com)

havasusteve03Beast · 12th March 2010, 08:24

Quote:

Originally Posted by doe

Thanks, found them for about $260
which is a good price.

Looked briefly for a guide to replacing, but it seems the process is basically to get under the car, find the old sensor, unscrew it & replace. Correct?

Doe,

You didn't say, but was that $260 for Pre-CATS or all 4 sensors (Pre-CAT & Post CAT's)? Amazon has the Pre-CATS for roughly $110 last time I checked. I would only replace the pre-CATS since they are the ones that have the history of going bad. Post-CATS dont go bad very frequently.

My 2 cents

Steve

gsfent · 12th March 2010, 18:54

Quote:

Originally Posted by havasusteve03Beast

Doe,

You didn't say, but was that $260 for Pre-CATS or all 4 sensors (Pre-CAT & Post CAT's)? Amazon has the Pre-CATS for roughly $110 last time I checked. I would only replace the pre-CATS since they are the ones that have the history of going bad. Post-CATS dont go bad very frequently.
My 2 cents

Steve

True, but I thought it was post CATS that measure the efficiency code by comparing the values from the pre cats to allowable values the post cat "sees". If the post cat sees too much flow (from say headers), then it gets a measurement outside the allowable range.

But for the OP, pretty unusual for BOTH post cat O2's to go bad at the same time and since you didn't trip a code after installing the revised exhaust system, I would look for a more common issue, as suggested above.
Regards,
Jerry

doe · 24th March 2010, 17:26

I cleared the codes and have yet to get another CEL. I still plan on replacing the o2 sensors sometime very soon though.