Coolant Issue

[CelicaJoe]

Hey, new to the forums. My girlfriend drives a 2016 1.8L Sonic. I’m semi savvy with cars, but usually primarily with my car as I work on it more often. I try to do any repairs I can on hers however, if it’s within my scope.
Earlier in the year she was going through fluid fairly quickly. Couldn’t find any leaks on my own, but I read on here there’s a semi common thermostat problem. Seeing as hers is a 2016 I thought that wouldn’t be it. Had it checked for leaks, ran on a lift for 2 hours with dye and they couldn’t find anything. I ended up changing out the thermostat and that wiped the code (P0599). Thought that was the end of it. Recently her light came back up because she was nearly dry on coolant. Filled it up and cleared the code and it doesn’t pop back up.
The only thing I can think of is the head gasket. There’s no white smoke in her exhaust and the idle is fine, so I’m not fully convinced. She doesn’t lose coolant that fast, but definitely consistently. I can only guess she’s burning it. I was running through threads on here for a few days and I can’t find any potential solutions. Any one have any ideas?
Thanks

 

[gnonne]

How many miles?

Head gaskets are not that common on the 1.8L engines, but the coolant must be going somewhere... How does the oil look?

T-stat housing are not semi-common. They ALL do it and if you get a cheap one on eBay they do not last very long. Maybe the housing was leaking again and it got low enough that you could not see it leaking?

Also common are cracks in the bottom of the plastic overflow tank.

 

[Z71]

Another possibility is simply cracked coolant overflow tank. Sometime they develop hairline cracks near bottom and you cannot see any coolant drips as the leak is so slow. I had this happen with my 1999 Mazda 626 V6.

The oil cooler on this car tends to leak coolant after so many miles.

Also, there are many source of coolant leaks. Suggest you track all coolant hoses and lines to their connections and check there. I think that even the throttle body is liquid cooled to so it will have a couple of coolant lines going to it.

 

[CelicaJoe]

98xxx miles on it. I got the thermostat from autozone, it was aluminum this time. I’ll check the reservoir for cracks. Glad head gasket issues aren’t common. She just changed her oil so I’ll take a look at it. Thanks for all the input, that gives me a few different hopeful answers to the problem