Had the same thing in my st25rb when the thermostat was in auto. I cycled through until it was in the off position, let everything shut down then toggled through to furnace. Only the furnace should come on.
As well be aware you may have to heat the furnace up to get ride of any residue. We had to set the thermostat to 85, open all the doors and windows and let 'er rip for about 20 min. otherwise the smell was very unpleasant and it set off our smoke alarm.
Home is where the hookup is.
2012 ST25RB Reserve tailgating a 2011 Chev Silverado 2500HD Duramax