I got the car to a shop which has a nice snap on tool.

on the tool we see about 0 on the STFT and about -8.9 on the LTFT reading whilst at idle - meaning the ECU is seeing that the resultant exhaust is rich. The LTFT even whilst on driving was the same.

I later tweaked the andriod app and see the exact same readings.

The mechanic said that I should firstly clean out the MAF or replace it. then retest. If no dice then replace BOTH O2 sensors with genuine delphi items. GM ecus are very quick at adjusting the trim, this sort of LTFT can cause the CAT to overwork and maybe the cause of the P0420 code. Says that he has seen a lot of cars with this issue.

sounds like a plan does it?

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)