Hello Peter,
I think I found a small error in your modification:

You wrote:

Temp2 = Temp2 / 10
Temp2 = Abs(temp2)
Print "T = ";
If Temp_c < 0 Then Print "-";
Print Temp1 ; "," ;
If Temp2 < 10 Then Print "0";
Print Temp2 ; " C"
Locate 1 , 1
Wait 1
Lcd Temp1 ; "." ; Temp2
Locate 1 , 5
Lcd " C Wohnraum"


I think should be:

Temp2 = Temp2 / 10
Wait 1
Locate 1 , 1
If Temp_1 < 0 then Lcd "-";
Lcd Temp1 ; "." ; Temp2 ; " C Wohnraum "

The rest can be deleted.

This was my first real attempt at writing a Bascom 8051 program so I don’t have any software, yet.
Greetings,
H.J. Windt