busted O2 doesnt mean - broken o2, a lazy or blind O2 is also a pain in the behind and can cause the cat to break down over a long period of time (running lean or rich - both effect cat life)
All that is known in Pakistani mechanics head is to "thraatal baady ki safaa karo, injeckturr phoonk ke safa karo, pump pressharr oongli rakh ke check karo, taput setting lathe masheen walay se karao - aur hamari mazdoori ke paise nikalo.
They dont give 2 hoots about the original problem.
let me give you a simple example I faced once on a toyota 2JZ-GE Crown which was guzzling fuel yet no check light but occassionally lit the CEL with P0420 code now and then (mechanics, dealers blamed it on fuel)
The problem was the fuel pressure regulator. heres how I found out.
I first hooked up a live graph plotter to see the exhaust activity, the post cat o2 activity, the LTFT and the STFT. The LTFT was going really low (cutting fuel out) yet it was running weirdly rich. I induced a vacuum leak and saw some difference but nothing great. I tested the air flow and it was also reacting good.
When I tested fuel pressure I saw it fluctuate on no reason, I then removed the FPR vacuum pipe and after about 10 minutes I saw some fuel leak out of the FPR vacuum port itself. That was the leak causing excessive fuelling. I replaced it with a brand new part, new fuel filter and everything came back to normal.
I wonder why fuel ka kameenapan, fail teknalojy, company faalt was blurted out. I had reasons believe no diagnosis was done.