Possibles; Either cat, lamda sensor or another sensor is in trouble, or the wiring to them. Have you had it on a diagnostic reader to see what fault codes are logged? This could save a lot of money wasted on swapping out parts that are working fine. does the engine temp gauge read OK?
Im not sure the marea relies on the one that you can see as mine sometimes flies up to hot and then back down to normal due to a poor connection somewhere without the engine missing a beat.