1995 Toyota Landcruiser service engine light

My dealer's computer analysis said the light was on because my catalytic converter was bad, which meant
I needed a new one plus a gasket repair, plus a
tune-up. After paying the $2,000 bill for that the next day the light was on again. Now the dealer says it needs
a new air flow sensor. They said the computer only tells them where to look, but does not identify the specific bad component(s). I thought the computers were
component specific. Is this dealer deceiving me?