Dockyard

My hawk eye had been in at the dealer getting a new engine!! In the mean time it had run out of mot so they r noting it!!! If failed on emissions co reading of 3 %
And found and air leak in the bov so it was replaced with a new one now the reading is 1.27% but it should be. 0.20% at fast idle.
Any ideas what might be the problem they recon it might be the Afr sensor or the O2 sensor so they are going to try that first and retest tomorrow but the lands reading is also quite low so would that suggest it's knackered as well!

Any suggestions might be a great help
Thanks in advance
I should say it has a second de cat and cat back exhaust.