Over heating/ lagging when accelerate ing

This is a common mistake. The diagnostic fault code did not say to replace parts. They never do. They only indicate the circuit or system that needs further diagnosis or the unacceptable operating condition. The code is just the starting point that tells you where to look.

It is more likely something is telling the Engine Computer to keep the EGR valve open at the wrong times. Your mechanic will also check if the radiator fan is turning on, and the actual temperature the engine is running at.

So no idea what it possibly could be.

You haven't provided any test results or clues I can use to form a guess. There's dozens of things that can cause an engine to run hot. I already suggested a dead radiator fan. There could be corroded cooling fins on the radiator, the cooling system could be plugged with the red Dex-Cool, (Dex-Mud) antifreeze GM likes to use, a cylinder head gasket could be leaking, thermostat could be stuck. It could even be as simple as low coolant level.