The decoder card is what makes the locomotive operate with DCC. When you remove the shell you should see a small circuit board mounted on top of the locomotive chassis.
Make sure nothing is hanging from the underside of the shell to make contact with the card, and that the card is pushed firmly onto the connector.
If it uses a harness and plug, make sure the plug is seated on the connector.
I don't think this has anything to do with the shorted condition, but make sure you have cancelled the consist with this locomotive if running as a single unit.
Deutsche-Bahn Eisenbahn modeling Upper Bayern, western Austria, and northern Switzerland.