I have a 1999 Audi A4 Quattro 2.8with 131K miles
Last month, the Airbag sensor came on, then went off.
Next ABS sensor came on andstayed on
I replaced sensor for about $1800

Now Airbag light has come back on, check engine light came on and off, and car hasa clicking sound...
Took to mechanic, and was told catalytic converters need to be replaced... cost $1800 (dealership wants 2100)
Diagnosis was:
Permanent DTCs 01>16806/P0422 035 Main catalyst, bank 1, efficiency below threshold
Intermittent DTCs: No Fault found

Why so expensive to replace catalytic converter? Is there a cheaper alternative to replacing the catalytic converter? I live in DC. Or should I get rid of car as it appears I am doing a lot of expensive repairs...

Any advice would be helpful...

Thanks!