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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.
 

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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.
You need a new downpipe assembly. Just got this on my 04 LJ. It turned out that one of the smaller cats on my jeep had puked its contents into the main cat. Your TJ has 3 Catalytic Converters. The downpipe assembly costs about $800 depending on where you buy it.
 

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I had a bad pre-cat on my '04 several years ago but I'm not convinced, yet, that you don't simply have a bad upstream 02 sensor. A bad upstream 02 sensor can kill your mpg by fooling the computer into setting the fuel mixture to full rich. Try unplugging that particular sensor and drive it around for several days like that and see what happens. If the mpg goes back up, it's a bad 02 sensor.

And you can read your own codes, you don't have to have someone use a code reader. Simply turn your ignition switch on-off-on three times within five seconds and then leave it in the on position. Within 5-10 seconds, if you did it correctly, any DTC codes present will display in the odometer.
 
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