my hummer 2005 started having transmission issues to where it doesn't want to go into 3rd gear. I heard it might have something to do with the catalytic converter. Anyone know?

 

Welcome to HF. Tech questions go in the vehicle sections....... H3 questions should go in the H3 section, so I moved this for you.

There are no 2005 H3s. First available were 2006, sold in late 2005.

Whenever a question is asked, Miles on the vehicle is important, as is any recent work done before the problem started.

Cat condition has nothing to do with a Transmission. Are we talking about a manual or an auto trans?