E38 LTFT banks 1&2 differences and p0420 code

[icetraxx]

Is it normal for the LTFT to vary consistantly anywhere from 6-10%. Monitoring them this morning I saw that while at a steady cruise one sided was reading -5.6% and the other side was registering -4.8%. When throttle was applied they jumped to almost even. The reason that I am asking is that I recently started getting the p0420 code. I can clear it and it will not appear for 2weeks to a couple of months. If I don't clear it, it will stay there?

[joecar]

LTFT variation in a small range like that is normal.

P0420 is a result of the catalyst monitor test failing the bank 1 cat...

after driving at 55+ mph for some period of time, when you slow down to a stop and idle for a few moments, the ECM briefly drives the AFR rich and then lean for a few seconds... it measures the response time of the rear O2 sensor wrt the front O2 sensor... if the response time is less than the threshold, that DTC triggers.

If the rear O2 sensor stops responding you won't get P0420 (you'll get the DTC for the rear O2 not having enough activity).

So P0420 means your bank 1 cat is losing it's ability to store oxygen and it's catalyst efficiency is getting worse (it probably won't pass the sniffer test).

[icetraxx]

thanks joe. I talked to my tuner about it. Since I am running the dynatech headers and cats, he is thinking I might just need to turn the rears off as it is sitting on the threshold of throwing the code. I can clear it and it might not appear for a couple of weeks or months then one day it just comes back.