If you follow the link procedure and the transmitter says that the link fails, but you still have control over the receiver, something's wrong in the receiver, transmitter, or both, Please make sure the transmitter is set to the correct mode (FASSTest-18 for the R7008).

Activating the appropriate sensors in the transmitter's sensor function (in the Linkage Menu), and then re-linking the receiver will activate the sensor. Receiver battery voltage sensing is automatic, and does not require a separate sensor.