Most likely, the tether cable between the locomotive and tender is not securely connected. Check the orientation of the pins and push the cable in firmly. Use some small tool if necessary. Be careful not to damage any wires when pressing the tether cable in. Another possibility is that there is something programmed into CV19 other than a value of zero. Try a hardware reset, or try setting CV19=0 on the program track in Paged Mode and/or Direct Mode and then move back to the mainline and test.