I recently was started my elantra and it barely started and ran really rough. It idled very high when at low RPMs. I thought maybe I had a transmission or misfire problem. I took it to autozone and I had two codes pulled on the ODB II. One was a speed sensor no signal and the other was a cylinder 2 misfire. The car is overdue for a tune up and I plan on changing the plugs and wires, hopefully that takes care of the misfire, but what the heck is the speed sensor?? Is that a transmission part and is this common or expensive to repair??

Thanks