Oh that ain't good!

 

Make sure the button isn't stuck. I had not used mine in such a long time that when I went to press it to just check it out one day it was stuck. I pushed it really hard and it broke loose and actually depressed. I guess a drop or two of coffee had spilled on it and had dried up. Ewwww

I check it more often now.

 

+1 to this. I got my '06 ~2 years ago and the OD button was "gooed" shut so hard I didn't think it was the OD button... until I read the manual. I'd guess that the previous owner NEVER used it.

As already said, it's very helpful in 2 cases 1) when towing and 2) when coming down steep grades when driving in mountains. Think of it like throwing out an anchor to help keep you from riding your brakes.