I don't remember exactly how to do it but you have to change some settings on the thermostat, I only know because ours did the same thing but that was two years ago, it was one of those dah moments.
"Well that didn't go as expected"
2015 Chev 2500HD Highcountry Duramax
Cedar Creek Silverback 33IK
Donald&Casey cairn terrier
Rest in Peace Mary my darling wife.
Scottish by birth Canadian by time.