Bad cats are hard to detect. They don't show much emotion. Dogs on the other hand normally have a guilty look on their face.

 

does the o2 sensor that goes into the cat effect gas milage?

 

That would be the front O2 sensor? If so, then, yes. It has a profound affect on mileage.

 

For 1994-1999 Ram 1500s, the O2 sensor going into the cat is the "upstream" or "pre-cat" sensor. As HeyYou stated, it has a profound effect on gas mileage. It "judges" the exhaust gasses, and tells the PCM whether its running lean or rich, and the PCM (with input from other sensors) adjusts the air/fuel mixture appropriately.

 

Can you disconnect the upstream sensor to see if mileage improves? Here in AZ my truck is subject to emissions testing so removing isn't an option.

I've done the kegger / throttle body mod, plenum repair, complete tune up, CAI, cat back exhaust, e-fan conversion, water pump, thermostat, timing set and still only got 10 mpg. ?!?!?!?

I'm hoping its the sensor. I realize mpg and 5.9 don't really go together but have read where some guys are getting mid teens. I was hoping 13 mpg to be realistic.

 

How is your cat? How old is your pre-cat O2 sensor? What brand? What size tires? What gears?

Might just wanna replace it with a new NTK sensor, reset the PCM, and take it from there.

 

Fairly simple. Bat cat, like bad chicken, has a decidedly nasty smell. If your cat stinks, you can be sure it is bad. Don't believe me? Just check out the dumpster behind most any "Chinese" restaurant.