Hi guys looking for ideas my car has just failed Mot emissions and am looking for your help as to possible causes.

Its a 1.4 16V AHW mk4 Golf 1998 with 100k

O2 sensor passes the test but CO fails @ about 0.60 and HC fails at about
260

New Cat fitted and new O2 sensor due to O2 low voltage DTC.

Any ideas??

Cheers Andy