B4: Temperature indication indicates too little

Hello dear target group, where can I still look to get my cooling water temperature displayed correctly? – Voltage regulator is tested / replaced, tank indicator is correct – Multifuzzy is new – Tank display istrument replaced – AI board soldered, sucked, everything doesn’t help. – Mass connection tested at Multifuzzy, is there still any plug connection between Muktufuzzy plug and AI plug that could be gammelig?