2003 gmc envoy SLT P0172 and P0756

Hi my friend recently asked to use my 2003 envoy SLT to pull a boat, in route to destination he lost power to the engine almost sluggish no power when gas fully pressed only sometimes when you light foot it and forget it when you heat up! I noted since then that it is in all gears although mostly in 2-4 I've purchased an OBD2 scanner cp9180 the First scan I got Codes 0172 and 0756, after clearing the codes I have the same vec operating symptoms but Only the P 0172 code returns I read a lot of your websites info on this code and it is all helpful but It seems now I'm looking at the possibility to repair this fault could encompass checking every major system in the car, and I'm trying to narrow my fault possibilities before that Can anyone help me with this specific flow of trouble shooting on this problem ?

you've got two codes and maybe two issues! how does the transmission fluid look? which engine in your Envoy?

Hi jeff, thanks for looking @ my post, the xmisiom fluid looks clean but maybe a little hi, if that would have an impact . the engine power and firing ( I THINK) has really debilitated in the last day or two exspecially when it's warm, I live out in palm desert ca most of the time where temps are 110 degrees daily! Oh and the engine is a 4,2 liter strait 6 cyl I believe, 4whl drive. I've ran many test on it since and I only get the P0756 code now!. It always seems to be in a higher gear than it should be right from take off and wants to slip into overdrive to make up the power difference. but if I accelerate slowly and don't let it go into overdrive it will eventually get up to speed. but if i let it use overdrive Or It gets to warmed up I have no power and have to Stop and let it sit for 5-18 min, I can give you readings from the Obd2scanner if that would help But it is new to me and I'm not real competent with all it test, lingo, and standard parameters the readings should be? Let me know what info you need I plan on going though the whole engine tomorrow morning to try and slowly eliminate some of the possibilities.

Envoys had lots of issues with the catalytic converters plugging up leading to sluggish performance but I'd doubt that would throw that code. I'd suggest a dealer or a good transmission shop for some checking at this point. If it's something simple they should be able to point yout o the best repair but rarely are transmission faults diy friendly