I checked the 2013 SM and that's all it said. Check codes, if thermostat is suspect pull it and give it a bath. Don't have a 2020 SM. I know on my Saturn the cooling system was designed to route as much heat to the cabin in preference to the engine. human first, machine second. On my Sportage it was the opposite, no coolant flow for the heater core until the thermostat opened. A block heater solved that one.Since it is not an electrical item, my guess is that "No", it would not show up on a "code reader". There may be parameters the thermostat would affect that could possibly be measured, but I can't think of a specific test for a thermostat without removing it and observing its behavior in water at various temps.