My daughter has 13 chevy 2500 she pulls a four horse gooseneck trlr, 6.0 check engine light keeps coming on diagnosis from shop says cats are plugged if i delete cats put flowmaster muffler what is the best and least expensive tuner to go with that so she does not get any check engine lights and help with mileage shes only getting aroun 7 now first gas engine truck ive had since 74 chevy. Some say just replace cats but would like to see her pick up some power and mpg thanx for some ideas would be greatly appreciated.

 

some backpressure in the exhaust is good for power and mpg. it creates a scavenging effect and helps it flow better.
I'm not a fan of tuners for gasers. they are $400 to $500 where i am and don't help much for normally aspirated engines.

I would be interested in your results when you find one.
my 06 2500 4 speed 6.0 with 4.11's got 12mpg imperial gallon towing 6000lbs 100% highway.

 

Is there emission testing where you are? Straight thru pipes should cause you to fail.