2009 aveo code po599...whats wrong?

Hi everyone, im new here looking for some advice...i have a 2009 aveo about 87000km... A few months ago my engine light came on...changed the thermostat housing...now light is back on....i noticed after a drive the engine is burning hot thru the hood...but my gage inside is nowhere near the red....i dont notice anything leaking... I did top up my coolant today but im a single mom..so im afraid to drive it and get stuck somewhere....any ideas of what the problem is? I dont want to be fooled by mechanic..thanks