Patagonia is tricky for this kind of thing because there are long stretches without much cell coverage or nearby repair shops. Between Bariloche and El Bolsón especially, you're in an area where towns are pretty spread out. The best thing is to prevent problems: check your car before you leave (oil, water, tire condition, battery), bring a basic toolkit and, most importantly, make sure you have cell signal or a GPS locator that works without data. Some travelers get roadside assistance coverage from national companies before they go, and honestly it's worth it if something happens.
Once you're out there, in towns like El Bolsón or Perito Moreno there are more or less reliable repair shops, but the problem is not all of them are quick or located on the route. If your car breaks down in the middle of nowhere, calling the national highway patrol (depending on exactly where you are) or the local traffic police can help you get in touch with taxis or tow trucks. Unfortunately it's something that happens pretty often in that area, it's not just you being paranoid; lots of travelers report long wait times. One tip: before you travel, ask at your hotel or the local tourism office where nearby mechanics are and get their phone numbers, so you don't get caught without info.