My 2014 Volt had the same symptoms (no cabin heat on battery), and was diagnosed with the same problem in early December.
The dealership told me the same day that the part (aux pump) is on backorder/not being produced, but should have the part within a week or so. In the meantime, I told them I'll take the car back until the part arrives. 4 weeks later, I hear nothing back from the dealership and so I drop by to see what's going on. Finally they tell me that the part is still on backorder with no ETA, and also mentioned that there were about 2 other Volts with the same issue waiting for over 2 months! The only thing they would do is give me a phone number for GM to see if I can "push" from their side.
I called GM last week and told them the situation. They needed some time to investigate and I finally heard back from them today (week #5). Same story: the part is on backorder with no ETA and there's nothing they can do either.
I find it hard to believe that GM is not producing a part for a car that is less than 3 year old! Especially reading the forums and hearing from the dealership, there seems to be quite a few other Volts with the same breakdown. Fortunately, it's a leased vehicle and it'll be someone else's problem in a few months. I love my car and I've had zero issues with it until now. It's too bad because I was considering buying a used Volt once my lease expires, but as of now, I can't recommend anyone buying a used Volt.
In the meantime, I'm forced to burn fossil fuels for heat.