1 Missing screw should not cause any problems except the cooling system mounting screws! If you forgot to screw your radiators on cpu or gpu the shutdowns can be caused because of overheat since your cooling system does not absorb heat from chip properly! Also shutdows can be caused because you forget to plug cooler connector to mother board or there is a loos connection in connector so when loaded radiators absorb heat but cant output it outside laptop and then thermal sensor shut it down. Also you could mess up with thermal paste put it to much or too little. You need to disassemble it again and check everything.
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