Stefan,
I get the Hi message when I start the digi without the food and pit probes inserted. When you insert them the Hi goes away and I start getting temp readings.
I normally have my probes and fan plugged in before I plug in the 12 VDC. If you've got the blower, pit probe, and food probe plugged in before you power up, I don't think you'll get the Hi reading. I don't.
I really like my digi and it's a fairly simple thing to use, but the installation and instructional manuals leave a lot to the desired. I almost needed a magnifying glass to read my manual. I think the font was about a 4 pt or something like that.
Guru is very good at answering email questions and has great support otherwise.