Do the post-cat sensors actually do anything?

M

MickM

Member
Been doing some research into the post cat oxygen sensors (Bank 1 Sensor 2 & Bank 2 Sensor 2)

It looks like both of these need replacing on my Zed but from what I can see the only use for these are to monitor the Cats and don't actually alter anything? The cost of replacing these is fairly substantial if they have no use!

Anyone got any thoughts on this?
 
Yea that's correct,

They monitor cat condition really...but as the cats wear they work in conjunction with the pre cat lambda's and alter the long term fuel trim via the DME..

They elongate the usable life of the cats..
 
There's all kinds of BS folklore about O2 sensors particularly removing them or fooling them after a cat delete. A lot of those tricks won't work on later models and a missing or defective sensor will throw codes (annoying) and fail an MOT. Does a post cat o2 do anything other than tell YOU if the cat is ok? Given the whole purpose of the cat/ecu system is to trim fuel, air and temperatures for emissions it would seem logical if it did.

An article which is quoted all over the place (some forums attribute it to a BMW technical document but not which one) makes sense, the last paragraph says the post cat 02 sensor is used by KAM (engine data monitoring) to trim the fuel mix long term, so if we believe a widely quoted technical article the post cat sensor does do something.

Full version courtesy of bmwforums.info http://www.bmwforums.info/ecu-diagnostics-tuning-and-coding-forum/1971-bmw-fuel-trim-lambda-lambda-control-including-ltft-stft.html

TBH running a fairly complex high performance car like a Z4 requires some realism about costs and maintenance particularly as they age (mines 13 years old now! :cry: ) bodges and shonky short cuts will simply reduce the reliability, enjoyment and value of your precious beast. I'd get the sensor replaced :thumbsup:
 
Brill, nice one for the info!

Never read so much conflicting info on a sensor before! but what you say makes perfect sense. I'm getting the car up on the ramps to replace the suspension springs all round so it won't take too much more to replace both of the sensors while its up there!

.... May cost a few £ to repair every now and again but smiles per pound it's a bargain :thumbsup:
 
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