Thanks Fred, thanks Greg for your thoughts!
I've figured out the reason now. The firebox was supplied with flicker electronics by the Bachmann DC decoder, which I replaced with an ESU DCC decoder. Apparently he doesn't have this special setting. But it also works well without a glowing firebox, which should only be active when filling the combustion chamber anyway.
Greetings from northern Germany! Götz

Hi folks, I have purchased a beautiful Bachmann 4-4-0 large scale steam locomotive, rewired and fitted with an ESU Sound Decoder 5XL. Everything works fine! But the firebox does not work (connection to AUX1 and AUX2). Who can help me??
Greetings from northern Germany! Goetz