2006 Citroen Other Catalytic Converter/CPU

Hi, recently the engine management light on our car has been flashing. The onboard display is reading 'Catalytic Converter fault' and also 'ESP/ASR not functioning'. We took the car to the garage last week and they fitted new fuel injectors saying it should remedy the problem. Initially it appeared to have done so, however after an hour the fault reoccurred. I looked online and found a product called Cataclean, used it as per the instructions and again the malfunction disappeared for a short time before returning. We went back to the garage and they said the Catalytic Converter needed replacing. They replaced it this morning and still the problem persists. The dash is showing the same faults, and the car is almost undriveable. There is almost no power when accelerating and when the car is idling it often stalls. We are at a loss as to what to do and the garage has said they have done all they can. Please can you advise me what the problem may be, and what steps to take to fix it so that we can have our car back. Many Thanks