I have a 2015 GMC Canyon crew cab with the 3.6 liter v6. It has 68900 miles on it and I have noticed recently that the heater isn’t working properly. It will only blow hot air out of the bottom vents if it is set to high. All the fan speeds seem to be working fine and if I set the heat to come out of the top vents the heat works at all the temperature settings and from what I can tell it appears to be working just fine even if I have the vents set to the mixed setting.the only other thing I have noticed is that the coolant reservoir clear to the bottom. I filled that two days ago and am still having this issue.
Check in the FAQ and see if that helps.
If you are losing coolant, it's either leaking it or burning it.
Why the down vote? You said you were losing coolant. This is an honest answer based on experience.