Mitsubishi Repair: over drive problem, co2 sensor, autozone

Question
Hello, and thanks.
My 92 colt summit 2.4 l engine has a problem with the overdrive. It is broken and can't seem to decide when to shut off. As long as I have the overdrive turned off the car runs  fine, but when the overdrive button is in the car cant seem to decide when to get out of overdrive.  When I first encountered this problem, a mechanic at a gas station I stopped at told me it was just a nylon ring that  was broken that tells the car when to go in or out of overdrive. Unfortunately that mechanic is several hours away or I would ask him. Also, the darn car has had a problem with stalling once it gets warmed up.  Once again, my mechanic suggests getting a new co2 sensor, but at a $200.00 cost I havent gone that direction. He did a little tweaking with the idle which seems to have mostly fixed the stalling part at this point, but the overdrive he can't figure out. Thanks again

Answer
Megan: First off you need to check around on that cost for the co2 sensors. You can get them much cheaper, just got to check around. But that is what it sounds like to me. Could be your should have your car coded, AutoZone will do this for free. Your computer could be sick, it reads what the sensors send it but there is a part in those computers that go bad. It then leaks on other things and just kind of messes the computer up. So it is not reading things right, so your car won't idle right. The tweaking of the idle will work for a while, but pretty soon it will return. You get the car coded and see what they tells you. But check around for those co2 sensor as you will have to replace it soon. Now overdrive could be a nylon ring. I would check the tranmission fluid too, could be your car is low and so causing some problems there. Could be a switch is bad too. But not sure how to check that one. You should get yourself a manual on your car, a repair manual and can get one at any parts store. It would explain a lot of things to you about your car and it should explain how to check about the overdrive button too. Hope this helps and good luck!