my transmission will shift into 1st and 2nd but wont even try to shift into 3rd? could someone help me?

 

Check your fluid. Over filling will keep an automatic from shifting. Make sure the engine is warm and running when you check. I assume this is an automatic. If its manual and it won't shift you need a clutch or trans rebuild.

 

i checked the fluid its fine

 

I could be wrong with this but I think the c6 needs good vacuum to go into 3rd.