Update 2- went this morning to do the heater core flush, popped hood and coolant reservoir was empty, popped open radiator cap and no visible coolant. No signs of coolant leaks anywhere in vehicle or under vehicle. I added approx 3/4 gallon of coolant and I now have heat in passenger compartment, still have check engine light for the coolant temperature sensor, but so far all fluid temps seem to be normal. Hope that might be the fix.