no gas mileage at all. might get 6 miles to gallon. what to check?

Safe to say the check engine light is on and it's not running right? (rough or weak) , hard to start? FIRST thing to check is PROPERLY scanning and watching LIVE data. NOT FREE 'Autozone' code reading!!! IF you must resort to guessing by code , then first , write down ALL DTCs present , then erase them and drive it till light (and code(s) return. Read code(s) as soon as possible after light returns. POST that code(s) and we can start with that.