1999 Questions for all other cars engine fault

Engine problem
1999 Questions for all other cars Engine Size unknown Wheel Drive Type unknown Manual 11500 miles

hi I have a 1.4 peugeot 206 1999
i recently had a mot and it failed on emissions so I had a cat and lamber sensor replaced with new parts
i then returned it to the garage. While they were warming the cat up the drove it 1/2 mile and the car went down on to 3 cylinders(im noy blaming the garage these things happen). I was advised to check plugs and change the coil pack, which I did but didnt make it any better. Iv had the cam belt checked and the valves also the compression test was fine aswell any ideas of what it maybe? Or even better how I can rectify the situation.
iv had the ecu read and yes comes up with faults but nothing major but I was imformed that the ecu was getting red hot!
i have no lights on the dash warning me of any fault
please help me!