Automatic Transmission no longer shifting into overdrive
 

We have a 2000 Dodge Caravan (4-cyclinder) with an automatic transmission. When driving on the streets, it shifts into over drive. But on the interstate, it won't shift into over drive it just says in gear 3 (i assume). it's been doin this recently. idk what the problem is. i know that our o2 sensor is bad. has been for a while and now our catyllic converter is going bad. also, the wiring harness that goes to the steering wheel no longer works (no horn or cruise control). i wouldn't think that to have anything to do with it. but the service engine light has been on for a long time now due to the o2 sensor we never got around to replacing. so what could be making it not shift into over drive when doing high speeds?

Just to verify, you're sure it has an overdrive trans. I know that the first run of Gen3 vans with 2.4 did not have overdrive. I just verified my statement. I see no listing for an overdrive transmission for the 2.4 in 2000 (US specification) are you in a foreign country? because overseas vehicles can be quite different.

Now if you do and you are not going into OD but you're pretty sure it's working at lower speeds, what might be the problem is the throttle is pretty close to WOT which would prevent 4th gear. What could cause this is a plugged cat requiring heavy throttle to maintain speed. Also if you have a plugged cat, you would have low manifold vacuum which would tell the computer to hold 3rd gear because the engine is under load.

However to properly diagnose the whole problem, the internet isn't the answer. As you can see, even I gave a couple possibilites. All the guesses will lead to confusion and unnecessary parts replacement. you need to use a rational and pragmatic problem solving approach to fixing this problem. Diagnostic service manual, appropriate tools and a scanner with data are crucial to proper repair. If you can't provide these necessary tools, everything will be a shot in the dark, you may or may not hit the solution.