Check the coolant reservoir cap...

[sdeeter19555]

I tried removing my coolant reservoir cap on my 2012 the other day; the top came off, but the guts stayed in the reservoir's neck. Getting the guts out was a pain, it was like the o-rings had dried out and were sticking to the reservoir. Took about 5 minutes to get out without damaging the reservoir.

 

Got a new one under warranty, but its worth checking before you get out of warranty. I tried to lubricate the new one with coolant before I installed it...hopefully that cures it. I couldn't tell if it was an updated part number...GM seems to use a different code style than Dodge (which is what I mostly know).