2001 Pontiac Montana Transmission Problem

Have a 2001 Pontiac Montana that refuses to go into overdrive. It will hit 1st-3rd gear just fine but after 3rd we’ll start turning 3,000 RPM’s and it will start “humming” without going into Overdrive. Hoping to avoid the $100.00 diagnostic. Any help is greatly appriciated.

Also when we brake in the final second before comming to a complete stop we’ll hit a “clunking sound” any thoughts?

All you can really do is check the auto trans fluid level, following the manual on “how to check” instructions. If the fluid level is OK and the condition and smell of the fluid is OK then you are looking at taking it to someone for an evaluation. Your regular mechanic or AutoZone store can tell you if there are any computer codes showing up.