Karl Chevrolet

I have had a reasonably good experience with Karl Chevrolet up to this point which makes my most recent interaction with them even more disappoining. I own a 2016 Gen 2 Chevy Volt and took it in to have an oil change and the 45,000 mile service recently. At the time, I also noted that the check engine light for a fault in the O2 sensor had showed up a couple of weeks back and went away on its own after a day of driving. This is an issue for which I had brought in the car twice perviously under the bumper-to-bumper warranty (once when the light was active at the time I dropped it off for service) and both times the service department told me that it was a false alarm and they just reset the indicator. It turns out that GM does not have a solution for this issue at this point and is working on a resolution, which is perfectly fine. However when I asked for this to be looked at again this time, the service department told me that it would be out of warrantly (because of the high miles) unless the issue is exactly the code for which I had brought it in earlier (the O2 sensor). The example the technician gave me was that if the wiring was faulty instead (ie no problems with the O2 sensor itself), it would take them the whole day to debug and fix the issue and this would be at my cost. This is absurd. The service department chose not to do this deep dive to check for faulty wiring both times I brought the car in for service under warranty. In other words they chose to not check to see if there was any specific fault with my car because there was a more general issue impacting all vehicles. How is it now ok for them to charge me if they find an issue that would have been covered under the warranty at the time? The simple fact is that the service department has no way of knowing if any fault they find now was present when I brought it in previously because they chose not to do a thorough investigation at the time. It is disappointing, this was one of the better dealers in this area. Maybe it is just time to move away from this brand.