You�ve already got loads of good advice here, what I want to add is that if
it�s recently smoke tested and the O2 sensor is tight, try to get the cat
off and shake all the loose stuff out !
You might even take a tireiron, a long screwdriver or something similar and
help the ceramic out of there !
Not too good on the environment but it keeps the Bus rolling without
clogging the muffler and you could save up the cash or find the best deal on
a new one  to replace it with before the next test....

Calle
-88 Volgon
Mullhyttan, Sweden