i build an K8400 3d printer. But the temp of nozzle 1 is always 20°C.
The output (LED an meas with multimeter) is always 0V and the LED is not on.
I have use the onboard elektronics it self (mini screen) and with the replicator application.
You mean replicator or repetier?
repetier host win application from your download page
Oh, just to mention, i’m not Velleman staff. 
I assume with nozzle 1 you mean the first extruder?
Is the heater properly connected?
Did you pay attention to connect heater and thermistor to the correct ports?
yes i mean the heater 1 and i have checkt the connection. on the mainboard no led ist indicating and if i measure it no power is on the connector (on mainboard).
How are you trying to heat up the nozzle?
Have you updated the Firmware with the version 1.1 for One head?
hi i have arduino on my computer and i can compile the firmware but the upload doesnt work.
how can i upgrade to the newer firmware?
thx ok im on 1.1 FW
But the display shows Err:MINTEMP
ok it was the wrong fw it was for dual head
but why deliver velleman a singel head build kit with the 2 head fw?
Most people install the optional 2nd head right away during assembly of the kit, so less people have to perform an update.