Loosing Patience MK1 VRS Combi might be for sale

Was thinking of getting rid of my MK1 Vrs combi a couple of months ago as the milage was getting hi (now 150k) but was still running strong with no real issues and so decided to keep and spend a little money on her. 2 new driveshafts 2 new front wheel bearings timing belt and tensioner and while doing this noticed the cat was split where the mesh part runs into the pipe. Bought a new cat from automotive online and this is where the problems started. Cat fitted no worries and was the correct one for a VRS as it was checked twice on there datebase. Within 50 miles the engine management light came on. As I have no code reader I changed the rear lamba sensor and cleaned the MAF sensor and cleared the faults. Once again within 50 miles its back on again. Car runs fine no hunting or misfires and still returns a good MPG. Will now order the front lamba and see if this changes anything but slowly getting very ****ed with it and just wanted to know if anyone has a similar problem that they sorted.