i have a 09 silverado with a 4.8l and a 4l60e trans. customer believes his tow haul modes do not work because he does not see a radical rpm change when shifting in and out. called tech assist and they said that if light is on and no codes that is indeed functioning. is there any way to see if it is in fact in tow haul mode.

Tow haul should lock you out of overdrive. If you are cruising in overdrive and hit the switch the RPM'* should go up.

http://www.chevrolet.com/content/dam...ado_owners.pdf
From what I understand of it, it keeps the tranny from hunting around. Just screwing around with the button it tends to make the tranny hang in a gear a bit longer, and it also opens up other features too that can be set. So, when someone is cruising along and hits the button, they probably won't notice a difference.

Tow/Haul mode enables the operator to achieve enhanced shift performance when towing or hauling a load. When tow/haul mode is selected, the tow/haul switch input signal to the body control module (BCM) is momentarily toggled to zero volts. This signals the powertrain control module (PCM) or transmission control module (TCM) to extend the length of time between upshifts and increase transmission line pressure. Cycling the tow/haul switch again, or cycling the ignition switch disables tow/haul mode and returns the transmission to a normal shift pattern. Watch your data in the bcm, pcm, and tcm when commanding it on.

im not sure they lock out 4rth either, the 2500hds we have at work still have 4rth just shifts hard and at higher rpm. hard to notice if you dont have a load