u/sicktouches

I got a check engine light on my Automatic 2016 Hyundai Sonata Limited (100,241 miles). I stopped at an Auto Zone and they tiled me the error is a P0128 Coolant Thermostat (Coolant temperature is below regulating temperature). My question for the mechanics is this; how do I know if the issue is with the engine thermostat vs the coolant temperature sensor. Looking for signs or symptoms that will help me narrow down the source of the issue.

The temp gauge seems to stay near the middle while driving, but maybe it’s just stuck like that? IDK, I’m just looking for help. Thanks in advance.