DELL XPS 15 9550 randomly shut down

I bought a second hand XPS this Oct. After that I faced several times of random shutting down. I have changed the CMOS battery and main battery but it doesn't make anything change. I also change the system cooling policy into active and change the thermal management into cool but the problem still exist. In the Power log of BIOS it shows "ASF2 FORCE OFF", The BIOS is 1.5.1 and I have updated the latest IME. Is there anyone can help me?