A little help with p0137 code

Recently had my r3 2009 rebuilt, street ported with d585 coils + dwell time map and decat sensor eliminated in the ecu so once decat is fitted it won't pop on.



I still have the original cat on the car from when the apex seal went and after the rebuild.



Driving back from the rebuild I got to 100 miles pulled into services to use the toilet, got back in and after pulling back onto the motorway the light popped on.



Checked once home after a further 90miles and code was P0137 was there, I cleared the code and drove for about 10miles hasn't popped bk yet..!



Any help on this would be great as if my cat has gone I will put my toyo decat on asap!!!



As far as I can tell that code is a low voltage O2 sensor or something but please correct if I'm wrong.



Also what I should do from here on like change sensor or car ect.



Thanks



Luke