Stand-up heating doesn’t want to start any more

Dear community, the heating of my ’04 V70 D5 was still uncomplicated. When I wanted to start the reactor last week, the familiar “fuel heating AN” flashed in the cockpit. After two seconds, the computer revised its specification to “fuel heating OFF”. Is the battery too weak (unfortunately three times empty) or is it on the controller? Do you have any suggestions for me before I go to the friendly one? Greetings & thanks to Eucher china-taxi