I have a 2003 2500hd with a 6.0. It has 140000 miles on it. Changed the transmission fluid and filter less that 20000 miles ago. This weekend after pulling an RV for about 120 miles the transmission got to where it would not shift out of first in tow mode. In normal mode it will only shift out of first if you let off the accelerator. Then it will shift normally through the rest of the gears. Any ideas what could be the problem?

 

I am not sure on that late of model, but does it have a modulator valve on the tranny?

 

I am pretty sure you don't have a vacuum control modulator, but I believe there is an electic operated solenoid that might just be plugged. Anything done other than draining and replacing the fluid? If not, I'd also suggest if you haven't the easy thing first, with the tranny at operating temp, and running check the fluid level.

 

fuel filter. my 04 dmax did this to me. wouldn't shift normal. only high rpms, if at all. really erradic. i thought it was trans. turns out just fuel filter. use a quality filter tho. the filter i had on mine was wix, but didn't last 10k miles.