should never fit a cat till youve found and cured the problem that caused it to go in the first place...if that hasnt been done then guarantee it will keep blowing.
if engine light is/has flashed then the reason it flashed will be instantly stored in the ecu....majority of cars it will stay there till its been read and cleared with a reader..
if it was me, thats first point of call...get it read followed up with an emmission test and readout...
one tip....always remember any fault code is a clue to the fault, not a definite reason for whats wrong.
for eg...code tells you oxygen sensor is not switching...doesnt mean sensor is faulty, it could be a wiring fault, could be injection fault, ecu fault and so it goes on...
finally, be aware that cheap cats are known for failing prematurely....a definite item that fits in the you get what you pay for catagory.
Is the car a petrol or a diesel? what engine/ capacity is it?
(i assume its a petrol engine) if its a petrol the engine has to operate at the correct air/ fuel ratio, otherwise it can contaminate or burn out the CAT, without fixing this 1st it will continue to need CATs (as wase has said)
Thats a good piont about sensors etc, the amount of times iv'e read on forums, that someone hase replaced a sensor (sometimes at great cost) because its showing a fault code, to find the new one shows the same code
Also, other things can contaminate CATs, like a worn engine