i had a code 67 with my high flow cat but the light eventually went off (and my 2nd o2 sensor is disconnected, guys at the speedshop said it wouldnt change anything since i had a code 67 already). recently the CEL came back on at WOT but whatever. im planning on getting a carsound cat soon, ebay has decent prices, less than $100 i believe. i dunno if the 2nd o2 sensor changes MPG's... i get 23 or so per gallon and its been that way since i put on the 18" wheels