Hardware Flashers:Problem Solving

De-Bricking

Signs you may be bricked:

PS3 turns on but instantly turns off (Behaves exactly like the YLOD)
Black screen
Black screen + No HD activity (No flickering light)
Corrupt display (Colours are wrong, static, colour spots)
Combinations of the above 

Potential Causes:

Actual YLOD
Syscon panic YLOD
Installing unpatched firmware (Not dehashing)
Faulty/Corrupt flash or patch
Bad writing of flash

The reasons are endless, but if considering your console was working fine before attempting to downgrade you should start backwards.

Syscon Panic (YLOD Symptoms)

The only way to keep the console running is grounding the Tristate (If it already is try grounding it elsewhere). If using the E3, use its SBE pad on its ribbon. From here the console will stay in a blank state and you will be able to re-write or re-dump/re-patch your dump to attempt a recovery.

Black Screen

This is either because your PS3 is stuck in the position of waiting for you to install the FSM (after patching the dump yourself and writing it) and you should simply plug in your PSGrade dongle and continue downgrading. Otherwise its generally because the PS3 is soft-bricked. Solving this resolves around re-flashing the patched dump as it could have simply failed at the writing stage. If you have for some reason lost your original dump you can simply make a dump of what you have now and patch it again (This may or may not work, depending on the initial cause of the soft-brick). Another suggestion would be to remove either the ground or 3.3v lead/s from your HW flasher.