OK, I live in Virginia. In order to renew your plates you have to go through an emissions testing process. I swear most of it just benefits the shop not all shops that test are a certified repair facility either! It’s a 750.00 threshold to spend to get the waiver and many shops won’t tell you that either. So my CEL comes on and goes off randomly. It's sporting P0420 - Catalyst System Efficiency Below Threshold (Bank 1). What are we talking? Upstream 02, downstream 02, both 02’s, or catalyst, or all three? Vehicle runs fine. My previous run with a CEL is the shop milks you for all your worth starting off with the small stuff and then hitting you for the big ticket items. I’m not interested in paying the repair shops rent just getting my wheels to pass the dreaded emissions with CEL off! Before I start playing duck, duck, goose on what part to start out with replacing what does the code really mean and why can’t a good shop tell the difference either before my wallet goes dry?