A catalyst does not burn out like a light bulb.
Most catalytic combustors that are returned are either destroyed by flame impingement, broken due to accidents or mishandling, or there is nothing wrong with them but fly-ash build up.
A catalyst can be "saturated" with byproducts of wood burning such as potassium. This is chemical saturation. The prohibitive chemical will fill the chemical "holes" that the gases normally use for reaction.
This process of saturation can be slowed by regular maintenance of the catalyst.
Saturation can take several years, most units will last for a minimum of six year or 12,000 operating hours.