ours was fixed just by cleaning the sensor. the sensor on our rio is not on the transmission but on the right front drive axle just behind the rotor. it counts the revolutions of this cog wheel like sending unit that spins with the axle. if you remove the right front tire and can see a small flexable cable coming from the fender to the axle ...your on the rigt track.. if not....disregard. remove the bolt that holds the sensor in place. a 10 metric i believe. than you will see a small brass sliver on the sensor. clean that sliver off. it can collect debris, tack from fresh asphalt, road oils and such and slowly quit reading the revolutions. give that a whirl.