Basically to answer your original question, these are a few things that can cause the YLOD:
Dead/Disconnected HDD
Cold-Joint between the CPU/GPU/RSX(If your console has one)
Physical Damage to the Motherboard
Dead power supply
Not enough voltage from the power supply (Failing Power Supply)
Jammed/Disconnected Fan (Not Confirmed)
Overheating
Corrupted NAND Flash
Bad Update
Short Circuit