Vaillant shows more than 50° c?

I have a Vaillant Gas Boiler. A year ago it didn't work and a lot of people tried fixing it. Now it does work, but the display shows 50° degrees and more, although I have the thermostat switched on 20° (and it is 20°, but the boiler makes noise like it's heating all of Luebeck) - in fact, my gas output has doubled. Can someone tell me, which buttons I have to undo?! It's costing me heaps of money! A thanks from Germany to the young man, who already helped me with my computer!