Changing cat conv may not have been needed. May need fuel pump or ignition components. Would help if knew your engine and if has been flashing any check engine lights. Please provide info and then we can go from there. If you were told needed a cat conv would like to know the reason the shop gave for this or was it just a guess?
Cat converter was just a guess. No engine lights come on,, Car just shuts off like you turned key off. Forgot to mention that ignition module was also changed. Not sure but I think car is 2.2 4 clylinder.
If has problem with fuel pressure and volume you will experience stall but typically when fuel pump has probs it will stall like coughing a bit before finally shuts down, like losing power a little bit at a time. If in fact shuts off like light switch then typically is spark related. Since module has been replaced then may need to get shop involved and hope they can duplicate and then see what is missing when happens.