Road tripper - overcooling - please help.
 

In the last seven days, I drove my 2002 Elantra 2500 miles on a road trip. Since yesterday morning, the check engine light has been on, and the temperature gauge needle has been quite low (1/8 - 1/4 reading).

Autozone said that the code (P0128) meant "overcooling" and was nothing to worry about, and happens all the time when the car experiences a sudden change in climate (100 degree, bone dry Arizona to 30 degree, soaking wet Kentucky).

I'm paranoid, so then I took it to O-Reilly's and they got the same code, but were more concerned. They checked under the hood for a good 10 minutes and the only thing he saw was that the coolant in the reservoir was a little low. He said I would be fine to drive to my next destination (200 mi) but then to get it checked out more thoroughly.

Important points (I think):
*Besides the temperature needle acting weird, staying low, and the check engine light, my car is running absolutely normally.
*No noticeable change in performance, mileage, or heating in the cabin.


Would O'Reilly's have seen if the thermostat was stuck open - is it fairly obvious if they were looking around in there?
I am in a rural area now with a family member, and there are no mechanics for about 50 miles (unless they are just little garages that do not advertise). Am I safe to continue on my road trip?



Thanks. I love my car and I'm sad that this is happening to it.

I own a 2001 Elantra. Took it on a trip to NYC from Albany.
When travelling through the cool of the higher elevations, the temp gauge dropped sudden
No CEL. But, I understood it as the colder air suddenly cooling the hot radiator coolant.
No problem with engine. And when back to regular driving, no issues.

If your car stays cool and has no heat then would suggest replacing the thermostat.
It really is the only thing that would cause this issue.
(Other than a rapid lost of coolant and or pressure)
And, if you are a DIY mechanic then even better..
Don't forget to check level of coolant.

Good luck

Coolant worked
 

I filled it with coolant and the CEL went off.

For now, just keep an eye out for any problems
Otherwise, enjoy the ride

Will do - thank you for your help!