Every heating issue I had with my T420 is solved by opening up the laptop and thoroughly cleaning with compressed air. It will stays good for a few years until the dust gathers up again.
I have opened up my laptop more than once and cleaned all the areas i can, but I still can't seem to get the temperature to go down like others claim. At this point I'm considering replacing my fan assembly and see if I have better luck but haven't gotten around to do so.