2000 Dodge Dakota engine light

I had my engine light come on when the gas tank was low. The truck acted like you pushed a clutch in and the rpm's reved up. I am told that because the light went off that they can not tell what the code may have been. The truck burns a QT of oil every 4000 miles. The tranny shifts hard 1st to 2nd unless yo ulet it warm up for a long time. Even then it still shifts badly.