Ok hopefully someone can help me out and I don't have to take my car to the dealership. I have a 04 volvo S60 AWD 2.5t. I bought the car off an individual and not a dealership. When I start my car it shows the gears on the gauge cluster P,R,N,D, ect... I can drive the car for roughly 10 miles and then the cluster shows --- but no gear. From this point it won't alow the car to kick down a gear when I press on the gas but still a lows it to go. I can put the car in neutral, turn the key off and turn it back on and the gears pop back up on the cluster also a lowing it to drop down the gears when pressing on the gas. About a month ago the belt broke bc of my power steering pulley being worn out. I've since then replaced the belt and pulley but havent drove the vehicle bc when the belt broke it threw a code that says "reduced engine proformance ". So here's my question, can I reset the ecm to clear the code bc my car won't Rev out over 4000rpms and it graduatly accelerates.

 

You need an OBD code reader to view then clear the codes. You may be able to get Autozone to view/clear the codes or you can rent a reader from some stores. With that said, I'm a bit confused by your write up so I'm kind of guessing here. You say you can drive a bit then the car will stop displaying the gear position and then lock you into a specific gear? I'd research symptoms for PNP switch problems for your model (Park/Neutral Position sensor which tells the ECU where the shifter is. Try rowing the shifter from park to Lo like 20x fast to see if that changes anything. The car may also be going into "limp mode" where power is reduced to avoid transmission damage. You may have an issue where the transmission is heating up and the computer is trying to protect the transmission. Check your transmission fluid level and color (black with burnt smell is bad...) by wiping the dipstick on a white paper towel. reddish brown is good, brown is ok, black is bad. If dark you may want to try a drain and fill - just make sure you use the exact transmission fluid spec'd in the owners manual for your model

 

I bet your transmission is on its last leg.

Newer cars have "life time filled" transmission, and fluid never gets changed. Good for the people owns the car for its first 100k, bad for people that buy them used because they'll need a transmission replacement soon.