-8 here last night!! around 4 degrees during the day and NOT the slightest problem.
If you don't have this in your coolant then I'd highly recomend calling HPC and getting it as soon as possible.
Please note I am not employed by HPC, any advice or recomendations I give are based on my own experience and are not necessarily the same as HPC's. First point of contact on any hardware issues should be with HPC