Sup guys first time posting. I've been looking around and cannot seem to find out which cat I have to replace. Car broke 200xxx needed the replace the engine with 80xxx because of compression issues in 3 banks. Goofing off with a lotus and reclined. 3200 for a new engine clutch and clutch master and slave cylinder. Apparently the 420 code was on there from the old engine with the compression code but the shop that did the replacement never mentioned the 420 code. After driving around 2 to 3 weeks the engine light comes on again and it's the 420 code. Shop says for an additional 800 they'll take the old cat and have it rebuilt. I live in western NY state area. I rather do it myself and replace the cat but any help or opinions ions will be greatly appreciated. Had the tl since 12

 

Bank 1 Firewall side rear sensor. Probably cat, but I'd replace the sensor and if the light
returns replace the cat, or use 2 spark plug non foulers with correct thread size (illegal),
but will keep the light off in most cases. Info can be found on google.