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      06-05-2022, 05:56 PM   #53
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Drives: BMW 335i
Join Date: Mar 2019
Location: Milton Keynes

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Originally Posted by CarAbuser View Post
Because on decatted cars the diagnosis fails and will keep running.

It can happen on catted cars that have bad O2 sensors or bad cats.

The diagnostic routine manipulates the lamba target and looks for a proportional change in both up and downstream O2 sensors. It also looks at the lag in response along with a change in readings between the up and downstream sensors to work out how healthy the cat is.

These tests only occur at cruising and sometimes idle conditions. Once the engine is put under load they stop.
Thanks for the input, looks like the rest of us were not aware of the test, makes sense

Can you advise the reason this would only present itself on certain occasions?
I assume it always does it, just the severity of the fluctuations change?
For example my issue was only noticeable(but maybe always present to a lesser degree) after flashing to stage 0,however I've experienced it on all stg2, stg2+ and stg0 maps regardless. And the only way I stopped it was doing the full write.

Finally getting hybrids fitted and getting a wedge tune, so this should be covered in the new tune, just interested to understand in a little more depth.

Thanks again for the detailed response, just what I was looking for.
Appreciate 0