Well, I think I know the problem but I want you all to confirm for me just in case. Today on the drive home from Santa Barbara to Sacramento, an hour from home, my check engine light came on. I have 56,600 miles on the truck and it has the 5.7L Hemi. After the CEL came on tonight, my mileage dropped about 2-4 mpg. I think it may be the O2 sensor. What do you all think? I am going to go to Auto Zone tomorrow and have then scan it and see what the code is (to late on a Sunday night). I was gonna pull the one fuse (I think its 26?) to reset the PCM but after I noticed the decrease mileage, I figured it wouldnt help anyway. Anyone think they have an idea?

 

ttt. anyone?

 

You already know that to make sure your assumption is correct, you need to stop by an auto parts store to let them scan the PCM for codes to make sure. Then go from there.

 

Well, before i could get to a parts store, the light turned off, however the mileage is still really bad. Can the scanner tell me what the problem was even though the light is off now?

 

Yeah, it should be logged and recorded. It takes 3 trips to make the CEL come on but if while monitoring it again it doesnt get 3 consistent trips the CEL will go off.

 

Sometimes it will sometimes it wont. Many times its just a gas cap that sets it off (if you had recently gotten gas)... Whenever the CEL Is set, try to get to a parts store or the closest gas station with a mechanic to see what the light is before shutting the truck off. (Some mechanics will be nice and do it for free, some will charge 20-100$ so dont spend too much when advance does it for free).
Bad gas mileage can be caused by many different things... an o2 sensor is definately one of them but usually if its bad the light will stay set.

 

Would my Superchip 3865 read the code? I totally forget about that!!

 

My bet is EGR, mine went out around 50k and gas mileage tanked, power tanked, and it smelled very strong of exhaust and fuel. Do a search, many write ups, and many stories of hemi EGR valves @ 50k.

 

i think just about every cel affects fuel mileage. At least P codes.

 

Scanned it with the 3865. It is the EGR! Damn!