Recently noticed that my gas mileage has dropped by around 3 miles a gallon and transmission shifted differently. Found out that grade braking is on by default now. If I hold the tow haul button for 5 seconds it says grade braking disabled and everything is find; gas mileage and shifting behavior. No CEL or anything, so question is- how could this happen? And can I change this in the tune? What tables would I look at?
TIA

 

Not a tune-able feature as far as I know.

Not idea what your issue is. I've never had grade braking engage without tow/haul mode on while just driving around. And if you think about it grade braking is engine braking and should not use fuel during engine braking.

 

Check your column shift wiring for the Tow haul button. They are known to have issues. The wires are wrapped in an orange sheathing.

You have to pull off the column clam shell off on the bottom of the column to see it. Maybe you have a short or a loose wire.

GM Performance :: View topic - Intermittent tow/haul

 

Yeah... tuning doesn't have an expiration date and doesn't randomly go bad. Input somewhere is bad.. and I believe tow haul functionality is through the BCM.