I had the check engine light come on in my 2000 Toyota Avalon XLS (85000 miles) and I took the car to autozone and my local mechanice both of whom diagnosed the error code as P0420 - Catalyst System Efficiency Below Threshold ( Bank # 1). My mechanic said this problem was common on avalons and attributed the reason to the catalytic converter failure. He said it would cost me $ 630 /- for parts + $ 100 /- for labour to replace. Which In my opinion is too high. Has anyone faced a similar issue before ? There is no effect on the performance or MPG of the car. What could've gone wrong and how much will it cost me to fix ? Is it really the Cat conv or the O2 sensor ? Incase its the cat, can I buy one from junkyard ? Also will it have any effect on the car in the long run ( besides failing the DMV Inspection) if I continue to drive without addressing this issue.