I have the same problem. Modded JDM STi with 2.5l. The car had a decat downpipe, replaced with RCMS downpipe with in built cat and car has been mapped specifically to pass MOT as a new registration in Cyprus.

Failed MOT due to high CO2. They were a car friendly place and did not register the car, hence no readout and do not have to go back to same place. I notice that both the original decat downpipe and the new pipe had a plug with a sensor towards the end which is connected Is this the cat on fire sensor? What is this? Could this be faulty and doesit have a bearing on the emissions on the 2.5l. There is also a plug up near the turbo before the cat on the new pipe which has no sensor.

Am i missing an oxygen sensor before the cat and hence failing?

Car is otherwise in perfect order

 

So is the sensor that is on the header pipe drivers side the important one ?
I have been looking at a set of equal length headers but they do not have input for this sensor so did not know if car would run correctly, Car has decat pipe already and is fitted with Apexi ECU so could the fact that this sensor would not be connected be dealt with by remapping the ECU to suit ??
Dave

 

yeah thats the important one and i think u'll need it no matter what ur running mate, the decat should have a plug just behind the turbo you can relocate it there.