Help and Advice please Code - P0141 and P0420

[bechdan]

Hia 2003 Yaris exhaust needed replacing which was done at an ATS garage, straight after we got code P0420 - so took it back and they replaced the Cat with a new one free of charge along with a Bank 1 Sensor 1 O2 sensor.
After this we got codes P0420 and P0141 !!
So I replaced the Bank 1 Sensor 2 which made no difference.

It was due for its MOT so I used a code reader to clear the code temporarily, it passed its emissions test and MOT no problems.

The mechanic suggested that if the code persists it could be an ECU problem, but I doubt this very much as it only happened after the exhaust was changed.

The car runs normally, the main reason I want to sort it is if there is another problem that would cause the EML to come on, we wouldnt know about it!

So....... what should I check next? or is it simply a case of the ECU being fussy about a new Cat and sensors?