I have a 04 gmc sierra 1500 4x4 extended cab truck, 5.3 L engine. And it won't shift into 3rd / overdrive. Previous owner had it fixed but it's acting up again. No check engine light.. Ideas?

Is it low on transmission fluid? If not its probably a bad transmission. You could have it flushed but I would take it to a REPUTABLE mechanic.

Thanks I'll try that!

Y is my 04 GMC Sierra 4x4 saying reduced engine power and bearly go when just I put it to the floor?

if it shifts hard in between 1-2 and does not have overdrive but does not seem to get much worse as you continue to drive it. The problem lies within the transmission just under the oil pan its a 20 dollar part called the 1-2 shift accumulator. if you have clunks in between 1 and 2 but still have over drive then you will need the shift solenoid. when the pan comes off you will need to do a filter and oil change in the transmission at the same time so that is another additional 35 dollars plus 5 to 6 gallons of dextron 3/5 so all in all 100 dollar fix and about 25 mins to change one hour if your a first timer. I have fixed 30 or so 4L60e and 80e transmissions its 99 percent of the time the 1-2 shift accumulator or the 1-2 shift solenoid. does not matter what motor this goes for all the 4l60 and 4l80 e transmissions,