The wireless controllers of Carrera Digital are pretty good. The tumb operated devices do the job, but if you deep discharged the batteries it is over. This happened to me. What went wrong?
When the controllers are not in use, you have to recharge the batteries from time to time. You have to do this at least every 2-3 months. If not, the batteries will be deep discharged and will be useless. Disconnecting the batteries is the way to avoid this.
To change the battery in the controller you have to unscrew the compartment cover. Remove the battery and unplug the connection. Connect the the new battery (part no. 20089823) and close the compartment.
Carrera has part-loaded the battery in the factory but it should be fully charged before racing the first time.