weird overheating problem

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i was driving my car on the interstate when all of the sudden i notice that the temp gauge was at H and the check engine light is on. so i pull over and open the engine lid and i saw that coolant is bubbling and coming out of the overflow hose on the reservoir. so i turned it off and wait for it to cool down a bit and start again to drive to the gas station to buy coolant. after i bought the coolant which by the way is already pre diluted no need to add water. so i drive it again and