2017 Hyundai Elantra — coolant is old by age but mileage is low

I'd want a measurement, not a hunch. Move a coil or plug on the 2017 Hyundai Elantra if it's cylinder-specific and see whether the code follows. You'll know quickly if you're on the wrong tree. This is from a 2017 Hyundai Elantra in Seattle with 72,000 miles. When does it actually happen? Parking it somewhere I can hear it better next time.