So what happened is I've owned my tj for a year now and I've never complained about mpg ( it's a jeep lol ) . I have averaged 15-18 mpg. This last week I've been getting 7 mpg... what the he'll lol. I got a p0421 code and got it scanned today scanner also said the upstream sensor on bank 1 heater circuit is no good. What do i do guys any help is appreciated. 2005 jeep tj ,4.0 I6 ,6 speed, 3.73 gears.