I have a hyundai accent. It runs fine except the "check engine" light
on ever since shortly after I bought. No big deal, I remember the
dealer said it was a oxygen sensor problem. I haven't pay much
attention of it since it runs alright.

A couple week ago I find my transmission would not shift the to fourth
gear. (I have automatic transmission). There is no noise or iregular
event other than that. I pressed the OVerdrive button, it didn't show
up on the dash board.

When I sent to a transmission repair shop, they said some damage
happend inside the transmission, and cost 1200 bucks to fix it.

One thing need to be mentioned is that the car does occasionally shift
to the fourth gear, and when it works, the "Overdrive Off" light show
up when I press the overdrive button.

Generally the overdrive working after I have drive a while and the engine is hot.

So I take it to another trnansmission shop for transmission fluid
change, I didnt mention any problem. Their policy is to test drive and
do check on transmission so they can warantee the service for 90 days.
Surprisingly, when I ask if the transmission is Ok, he said it work
Fine. They did give me the code for the check engine light, which tell
me they have performed a diagnosis on the car.

I really doubt whether the first shop have actually did a diagnosis on
the car since they don't give me any code. And all the check up is
free.

So anybody know the problem is electronically on the engine's part, or
something wrong with the transmission's hardware?

At one time before I drove, I pressed the Overdrive button, it show up
on the dash, but when I drove away, and test the button again, it the
overdrive light wasn't working anymore, and can't shif the fourth gear
either.

It seem like when the overderive light is working, I can shift to the
last gear, but when the overdrive light not working, I can't.

Thanks folks for the helps.