I've recently purchased a 2009 R3 with 34k, engine has less than 10k, recent good compression test, BHR ignition, recent wires/plugs, catless header and resonated midpipe, runs and drives great. The previous owner already eliminated the CEL associated with P0420 due to the decat, but I've been getting a persistent P0139 as well. I've already been through the procedure in the FSM and checked the common stuff like exhaust leaks, intake leaks, cleaned the MAF and ESS, and replaced the rear O2 sensor with a new Denso just to be sure. So the question, with an otherwise great running car, is the P0139 CEL due to the lack of cats?

 

P0139 is typically what you get when you try to use a de-fouler or spacer to trick the ECU into thinking that everything is okay after a decat.

Basically, the ECU deliberately cycles through lean/rich in a specific pattern. If the rear O2 doesn't respond the way it expects, it throws P0139.