Check the speed sensor located on the transmission tail shaft housing. The sensor may have failed or the nylon gear may have sheared. If the gear is damaged,check to see if the sensor spins freely. If the sensor spins ok, replace the metal gear on the output shaft of the transmission also. Moisture may have caused the metal gear to rust. This in turn creates a rough surface for the nylon gear to ride on, causing the gear to wear down and fail. After the repair, pull the EFI fuse under the hood for about 20 seconds. This will clear the codes stored in the ECU.