If the garage do not have the code, this means they were taking guesses!
Absolutely! Guessing can be expensive. In this case they've guessed 'backwards' from an expensive catalytic converter to a (relatively) inexpensive lambda sensor and still haven't got to the bottom of it.
Catalytic converters can fail prematurely because of over-fueling/running too 'rich', burning oil, ('fouls' the catalyst materials) a 'jarring shock' (such as in a car 'accident' or a careless garage technician hammering at seized nuts/bolts on the exhaust), abrupt cooling from very hot (such as fording deep water after hard running causing metals with diverse rates of expansion and contraction to stress/crack) or a 'push-start' typically for a manual gearbox car with a flat battery when excess/unburnt fuel can get dumped into the exhaust system. Or a 'bad luck' manufacturing defect.
However, barring any of that, it'd be odd for a cat to fail at your car's age and mileage.
CODES are a must...
