It's perfectly capable of getting and staying up to temp now
I think I know what happened
Ice/debris/malfunction of the grille shutters sent it into some kind of panic/default limp home mode, keeping the fan on full blast, blunting warm up and heat performance
Or, it's an intermittent issue (position sensor/motor) issue with the shutters, that only manifests in severely cold weather
I've been able to open and close the shutters with bidirectional control many times, with no issue
The coolant being 8oz low was unrelated IMO 🤷♂️
I've finally got an appointment at the dealer (mid February 🤨)
I'll keep an eye on it, as temps have been moderate for this time of year (40° 🌡️), I'll mention it all to them when it's in
If it starts malfunctioning again in colder temps, that may agree with my hypothesis of intermittent shutter jamming
Edit: I just found a TSB
Condition: Some customers may comment on a loss of cabin heat.
Cause: The cause of the condition may be a software anomaly in the engine control
Correction: Reprogram the ECM.
It covers a lot of 2.0T LSY vehicles, and my build date XT4 falls into it
...GM, get it together 😒
I'm just gonna walk in, and hand them the TSBs, let see how insulted they get 😒