2007 Peterbilt 379 Isx coolant boil, code sta15

I gotcha guys. I will put the new cap on before I run a bottle test and hope it's a simple fix. I will have all day tomorrow so I will do that and run to pick up an afternoon load (30 miles) that should get hell ##### good and warm . She doesnt overheat bad at all in the cold and never really smell the coolant or hot smell of radiator from front with fan on . But just can't idle if it's slightly warm outside or it climbs . It never used to do that I ran her night and day for a month straight and never overheated or got warm really with fan off. So I know something is a miss. Maybe simple I hope . I don't want to do the thermostats obviously if it's something simple so I'm hoping it's a cap, fingers crossed. So I'll go cap then bottle test then thermostats I suppose about as far as my knowledge goes. Appreciate the feedback guys.