what function exactly does the lock up solenoid inside an auto tranny perform. I get that error code from the o/d dash light and I wanted to know what effects does it have and does it kill gas mileage at all.

2-- I also get an error code for my speed sensor. My tranny was replaced when I brought the car and I do not know if that has something to do with all these codes popping up. I looked to see if they did not connect something back but it looks o.k. Help me out. DOes an error with the speed sensor kill gas mileage as well.