Maybe someone more knowledgeable can verify this but aren't there some tests that can be run on the o2 sensors that while not always definitive might point in some direction? like check the live sensor data to see if they are flat lining or maybe there is a self check test that can be run under Rheingold for the catalytic converter like on the diesels?

if you could get Rheingold im sure you would have more to go off of rather than just what they tell you.