hp prodesk 400 g7 small form factor pc

[baratrônico]

Hello everyone,

I really need your help with an HP ProDesk 400 G7 SFF that became unbootable after a BIOS repair attempt.

The machine originally had a BIOS password. Before doing anything, I made a full dump of the 32MB SPI flash (GD25B256).
Unfortunately, I didn’t realize at the time that my dump came out corrupted in the BIOS Region (Descriptor/ME/GBE seem fine, but BIOS region is bad).

Then I flashed a full BIOS dump from another HP ProDesk 400 G7 (similar model).
After flashing, the PC would not boot anymore. No display, no beeps, nothing.

Now the system immediately shows this error as soon as I plug in the power, even without pressing the power button:

5 long red blinks + 7 short white blinks
(repeats continuously)

I tried flashing back my original dump (the corrupted one), but the behavior is the same.

So far I tested:

• With my original corrupted dump → still 5.7 blink code
• With the BIOS dump from the other 400 G7 → still 5.7
• With the SPI chip completely erased/removed → still 5.7
• CMOS reset → no change
• Win+B BIOS recovery → no reaction
• Powered on without CPU → still 5.7

It looks like the PCH is stopping immediately due to Boot Guard / HP Sure Start firmware authentication failure, before POST even begins.

Right now I have:

• My full corrupted 32MB dump
• A working 32MB dump from another HP ProDesk 400 G7
• All original system stickers (Serial, SKU, FeatureByte, BuildID, Board ID, etc.)
• Motherboard part numbers (M87686-001 / L76448-002 Rev 0K)
• Photos of the board and labels

I kindly need help with:

1. Rebuilding or repairing a clean working BIOS for this exact model,
   using my corrupted dump + the board information,
   OR using a clean/universal BIOS known to work with this board ID.
2. Confirming whether the 5.7 blink code on this platform is recoverable through BIOS reconstruction,
   or if Boot Guard makes the board permanently locked.
3. Any known procedure to restore/reset Boot Guard / SureStart on HP ProDesk 400 G7.

I will attach:

• Corrupted dump
• Donor dump
• All sticker photos
• Board details

MOD EDIT: Serial number BRJ3075HTH

Thank you very much for any help. I really want to bring this motherboard back to life.

[SMDFlea]

Hello everyone,

I really need your help with an HP ProDesk 400 G7 SFF that became unbootable after a BIOS repair attempt.

Thank you very much for any help. I really want to bring this motherboard back to life.

try this bios.

[baratrônico]

Thanks a lot for the help so far, guys — I really appreciate it.

I flashed the clean BIOS that was sent, but unfortunately the system still doesn’t boot.

Just by plugging in the power supply (without pressing the power button) the PC immediately shows the same error pattern: 5 long red blinks + 7 short white blinks, repeatedly.
It happens even without the BIOS chip installed, and even with CPU removed, so it seems the failure occurs before any real initialization attempt.

From what I've learned, this pattern seems related to a BootGuard / HP SureStart firmware integrity failure, where the PCH rejects the boot block before releasing SPI clock. That might explain why the LED error appears instantly after applying power and why changing BIOS images doesn’t change the behavior.

What I've done so far:

Extracted the original BIOS dump before flashing another chip (but the BIOS region appears damaged in that file)

Flashed BIOS from an identical ProDesk 400 G7 → same blink error

Flashed a clean BIOS provided here → same blink error

Verified power rails on SPI chip (all present except CLK remains idle)

Tested with and without CPU → same result

Tested with and without BIOS chip → same result

At this point it seems the PCH may have "locked" to a bad BootGuard state and refuses to initialize with any firmware that doesn't match the fused OEM key/signature.

What I need help with

Has anyone here successfully recovered an HP ProDesk 400 G7 SFF after triggering 5.7 blink due to a BIOS flash?

Specific questions:

Is there a known factory clean image that works with virgin or mismatched BootGuard state?

Has anyone fixed this issue by rebuilding BIOS using original Boot Block + new BIOS region?

Is it confirmed that this model uses unique per-board BootGuard keys, or shared keys across units?

In worst case, does replacing PCH actually solve it, or will it remain locked without the correct OEM provisioning firmware?

Thanks again!