That light sequence indicates a memory problem. Try reseating the RAM and if that does not help pull one of the RAM modules out and try again. If that still does not help try the other RAM module by itself. Have you recently added RAM or possibly installed a new kit of RAM? I have a E6420 and recently had to replace the CMOS battery with a brand new CMOS battery due to a memory error issue, that may also help. And if you did in fact add new memory before the problem started then replace the CMOS battery and leave the old one out for 15 minutes before installing the new one.
Good day. My Laptop Latitude E6420 do switch off after 3-4 seconds of switching on. With the blinking power signal
Latitude E6420 Switch off