Studio XPS 1647 laptop stuck at Dell splash screen + Volume DOWN LED flashing in PSA

Can someone tell me what the following PSA flashing error code means?

Symptom:

Studio XPS 1647 laptop can't boot past POST and stuck at Dell splash screen with progress bar at ~80%. After holding down FN and pressing the power button, all three volume LEDS would flash twice, immediately followed by the volume UP and DOWN keys flashing, and then followed by just the Volume Down LED flashing.

Recently my Studio XPS 1647 has had overheat problems, and sometimes they would cause BSOD with broken screen images. The BSOD always came with bcode 116 error: this is apparently video card related. So last weekend I opened the case, removed the heat sink and the fan. The heat sink vent was completely blocked with a thick layer of dirt, so it's no wonder for overheating.  I cleaned up the heatsink vent and the fan, cleaned up the top of CPU chip and the top of GPU chip, reapplied thermal paste and put everything back together. After that, the machine couldn't pass the Dell splash screen on boot.  I've already checked the discussions here and followed suggestions on holding  down the power button to remove static, reseating the memory card etc., but they are of no help.

Finally I followed the instructions on starting up ePSA. I've tried it repeatedly, and every time the boot always stuck at the Dell splash screen with the progress bar at about 80% with or without applying ePSA. The SPA screen never came up if I held down FN+Power button. The difference between a generic boot and a PSA boot is that if I held down FN+Power button, all three volume LEDS would flash twice, immediately followed by the volume UP and DOWN keys flashing, and then followed by just the Volume Down LED flashing.

I've read from a few discussions talking about all three volume LEDS flashing would be a motherboard problem. But in my case it's three LEDs flashing twice, followed by volume UP and DOWN keys flashing once, and then followed by just the Volume Down LED flashing continuously.  Does anyone know what this PSA blinking code means?

Thanks.