Ukoliko si sam minjao bateriju, skini backplate pa pogledaj gdje se točno grije.
Ukoliko je u pitanju baterija, onda ima loš kontroler i previše se struje gubi.
Ako kažeš da je to i prije bilo....
Google u moru "pametnih" savjeta uglavnom navodi ova riješenja:
Issue: Overheating
A few people have been complaining about the OnePlus One overheating. It’s worth remembering that any smartphone is going to get hot if you play intensive games on it, but if your phone is getting so hot that apps are crashing or it’s rebooting itself, then you may have a more serious problem.
Workarounds:
Take your case off.
Take frequent breaks to let it cool down.
Potential solution:
If it’s heating up and you’re not running anything intensive, then try rebooting the phone. A rogue app, theme, or anything else you’ve installed could be to blame. You can try removing things to see if it makes a difference.
If your phone still heats up, then clear the cache or try a factory reset.