Here comes the fun part.
The Biqu B1 has all it's wires to the hotend via a USB-C cable. All wires from the motherboard goes to a card in the back of the printer, and fom there is a USB-C to a card in the hotend. This card splits up to TH0, CNCFAN, CNCAN1, FAN2 and HE0. Even power to the nozzle heater goes in the USB-C.

On the old SKR v1.4, the wire labeled DCIN were connected to FAN1 (GND - 12/24v), and FAN0 were connected to FAN0 (2.3 - 12/24v).
On the new SKR MINI V3 the DCIN wire is connected to FAN1 (12/24v - PC7), and FAN0 wire to FAN0 (12/24v - PC6).
The wire DCIN is not from PSU, but gives power to the card in the hotend.

In the printer.cfg all of fan-stuff are commented out, and the only thing is:

[fan_generic hotend_fan] #use for testing of hotend fan pin: PC7
pin: PC7

[fan] #part fan pin: PC6
Pin: PC6

This gives me sliders in Klipper for PC6 and PC7, and I can set FAN1 to 100%.

This should in (my) theory give power to the card in the hotend. But when i slide FAN0 in Klipper nothing happens with the fans.

When i disconnect the wires to the nozzle heater from the motherboard i now have about 24 volts on those wires, and this happens without heating the heater.

The reason for me starting this project was that one day i just got error message when heating the nozzle. First i bought a new heater, thermistor and the block they are mounted in. This did not help. So, now the motherboard is replaced and I've added a Raspberry PI with Klipper to the setup, and still no joy.