fan turns fast and after that lcd gets dim

hi.

after a week this laptop came back and now i findout thats fan works hard temporally and loud after that lcd brightness play up down and get dim.

i used core temp app and hardware tester but no error showed up.after using them and put the cpu and ram and video under persur this not happening soon to soon.

its plugged in without battery may it can do that?

i think maybe heatsink not workd good like past.

its coming back with fn+f8 only.

tell me what you think?

thank you for your f1!!!