A little back story, this is a 2015 Jeep Wrangler. Since 2017 I've kept an eye on coolant levels because sometimes I can smell burning coolant when I'm in front of the vehicle (not in the vehicle). However, prior to last week I've never added any. I never needed to.
Last week I got a check engine light, with no drivability symptoms. I pulled the code and I got "ECT low". After a bit of research, I learned that my Jeep has a 203F degree thermostat yet it was running at 181F (steady-state). I verified this reading with a scan tool while driving because my first thought was that if it was low on coolant, air pockets could make the sensor read erratically, but the output in the vehicle information may just be an average. I also checked the coolant, I maybe added a pint. Considering this was the first time I added coolant in 5 years, I don't think it's low. When the engine is cold it's right at the max fill mark, when hot, the overflow bottle is just about empty.
I thought about replacing the thermostat housing, but I cannot surmise a reasonable explanation for how a mechanical thermostat could be the cause of a lowered operating temp. Usually, they are stuck open or closed.
My other thought is the battery. I bought a Optima red-top in 2017, but the date on it was 2016, so it's over 4 years old. I was debating if an aging battery could contribute to a bad reference signal.
Any other thoughts, or troubleshooting suggestions? I'm running out of ideas.