Won't shift into 3rd.I changed the filter and fluid and ran some stop slip through it but didn't fix it. Check engine light came on right when problem starrted. Googled it and found that 3&4 shift solenoid could be burnt out, which explains the light bein on. Just wondeerin if anyone else has run into this. Its a 350 vortec with 4l60

 

i have a 350 5.7l i thought the same its just my transmission output shaft seal you will be going and it will get into third then go into nuetral like reving really high is that your problem

 

That's exactly what it does. Did it make your check engine light come on? I just don't see how that would keep it from goin into 3rd but that's exactly what its doin

 

the transmission builds up the compression then and cant hold it cause the seal is bad and then it revs real high yea im sure that is your transmission output shaft seal replace that. also dont for get trans fluid and something to catch it with when u take the shaft off from u uv joints ALSO LOOK CHECK YOUR TRANNY FLUID IF IT IS GOOD TAKE IT FOR A DRIVE PARK IT AND LOOK UNDER THE TRUCK ON THE DRIVE SHAFT AREA FOR A LEAK? THATS WHAT MINE DID

 

it also could be a line to check that area also.