I've had my thermostat replaced 3 times on my 2012 sonic and each time after a month or so the code comes back up...p0599 I'm at a my wits end trying to figure this out...any ideas

 

Has the thermostat actually tested bad each time it's been replaced? Does the engine heating/cooling work ok besides the check engine light?

You can inspect the connector closely, possibly coolant or oil getting into it or other damage. Otherwise you might have a pinched or damaged harness, causing an intermittent short.