My ? would be are you using towhaul when towing ?
If not then start using towhaul when towing, Allison will suggest the same.
And yes, try to get the TCM codes read.
What's actually wrong
don't know until you get the code numbers.
And the repair cost, same answer as above.
2007.5 GMC "Black" Sierra SLT 3500HD SRW C.C. 4X4 LMM Z71
DIY stock airbox mod, aFe ProGuard 7, OEM drop-in
EFILive ECM ATP - Heavy Tow Tune with Turbo Brake
EFILive TCM Duramax Tuner
DHD Cold Side High Flow Intake Pipe
DHD PCV Re-route
2007 GMC Yukon Denali "BLACK" 6.2 gasser, DIY stock airbox mod, K & N OEM drop-in, Airaid intake tube, EFILive TCM tuned
Last edited by D_R_C; 04-28-2019 at 04:29 PM.