Transmission going bad?

I have an 02 gt with 125k miles. I had the trans fluid changed twice. The car seems to be shifting pretty sluggish and when I get onto the freeway it doesnt seem to want to shift into overdrive right away. Could this be the trans going or could it simply be an input or output speed sensor?

How is the Engine running?
Is there any Check Engine Light (CEL) on?
Poor deliverabilty of an engine can give false symptoms to a good transmission.

The engine is running great, no codes.

How does it react when you put the pedal to the metal, or WOT?
Is it slow to react?

It slowly shifts into the next gear. My grandpa would be happy with it! I was just wondering what the input and output speed sensor do and if they were easy to get to. They're fairly inexpensive so I was just curious..

Are you sure that you don't have a transmission oil leak?
Half a pint lost is enought to make it slip, causing it to have issues accelerating.

Did you check ATF with the engine idle and in park?
Dose it feel like lost horse power at the rear wheel?

Horsepower seems normal. I don't think it is leaking, I would've noticed it on the garage floor. There is no dipstick to check the level, I just assumed the transmission shop that changed the fluid filled it properly. I did add some lucas stop slip but it didn't feel any different.

If ATF was changed twice, have someone else check the ATF again.
The shop might be wanting you to pay for a Transmission repair.

I had the atf and filter changed for maintenance reasons. Months later I noticed it was seeping out around the bolt holes because of rust. I took it back to the shop and they said to order a new pan and gasket from GM. Since I work for GM that was not a problem. I bought a new pan, gasket, magnet and bolts. They gladly put the new parts on and it looked like they filled the fluid with the car running on the hoist. They didn't even charge me for doing it. I haven't been there in a while and they never suggested a rebuild. Is there a way to check it in my driveway or do I have to take it to the shop? I'm also thinking about changing the input and output speed sensors if it's not too difficult of a job. I can tell the previous owner was in an accident on the front drivers side. The car never seemed to idle smooth since we got it and i've changed every sensor expect the cam and crank sensors. Just recently replaced the head gaskets and put on a high flow cat and borla exhaust. That really woke the car up! Are there any products on the market that will let me adjust the firmness of the shifts?

there is a bold on one side of the transmission to remove to check the oil level. I don't know where it's located exactly, but a good service manual should show it

I know there were likely some changes to the design, but I had a 1996 that I changed the speed sensor and it was pretty easy to get to. If I recall I jacked the car up and took off the tire and was able to easily reach it.

Does it just have trouble getting into overdrive or does it not go into overdrive at all?

Under normal driving conditions getting on the freeway it will take a couple miles until it shifts into overdrive. Also if I floor it the shifts are very mushy.

I would say there is a fair chance it could be your torque converter.

Under normal driving conditions getting on the freeway it will take a couple miles until it shifts into overdrive.

Any Gm products that I owned and Still own take a few miles of highway running to shift into OD. It is winter now and takes even longer, so I don`t think that is out of the ordinary for your car.

It is true that the torque converter lock solenoid can take a long while before kicking in. The engine must reach it's operating temperature, and I think the transmission too.

Maybe you should check if the 2 temp sensors work correctly: engine coolant temp sensor and transmission oil temp sensor...

I would also look for trace of leaks for the transmission. That include the transmission pan, transmission cooling lines and the radiator... Also check in the overflow tank for oil in case the pipe broke inside the radiator.

If you have an RPM gauge, look for unstable speed, that could indicate that it slip.

FYI courtesy Xeroinfinty:

http://www.quality-trans.com/faq/faq8.html#41