Dell G5 15 5500 (DP/N 03GNH6) - Auto power on, no POST, black screen - BIOS not accessed?

[Mize27]

Hi everyone,

I'm looking for some help with a Dell G5 15 5500.

Service Tag: 6044B63
Mainboard DP/N: 03GNH6
Board markings:
SELEK MLK N18P
19795-1

Symptoms:
- Automatically powers on when AC adapter is connected.
- Black screen, no Dell logo.
- Keyboard backlight turns on.
- Fans spin briefly and the board continues attempting to boot.
- No response to the power button.
- Boot loop continues even on battery power until the battery is physically disconnected.

What has already been done:

BIOS:
- Backed up original Winbond (8MB) and Macronix (16MB) SPI dumps.
- Dumps were read multiple times and verified identical.
- Several experienced members here provided cleaned/rebuilt BIOS files.
- Flashed:
- Winbond only
- both SPI chips
- another complete BIOS set from a different member
- Every flash:
- Erase OK
- Blank Check OK
- Program OK
- Verify OK
- Absolutely no change in behaviour after any BIOS combination.

Hardware checks:
- Display cable checked and reseated.
- RAM tested.
- PCH pressure test while powering on:
-> No change.
- PCH gets warm.
- CPU gets warm.
- NVIDIA GPU gets warm.

Voltage measurements (board powered on):

CPU VCore (R10 coils):
~0.8-1.0V stable

2R2 coil:
2.92V

R47 coils:
Left: 1.20V
Right: ~7V

USB VBUS:
0V

Resistance/Diode checks:
No obvious short found on the main power rails.

SPI flash measurements:

Winbond W25Q64:
Pin 8 (VCC): 3.3V
Pin 4 (GND): 0V
Pin 2: 3.3V
Pin 3: 3.3V
Pin 7: 3.3V
Pin 1 (CS#): ~2.66V
Pin 5 (MOSI): 0V
Pin 6 (CLK): 0V

Macronix MX25L128:
Pin 8 (VCC): 3.3V
Pin 1 (CS#): 3.3V
Pin 5 (MOSI): 0V
Pin 6 (CLK): 0V

(I know a DMM cannot properly display SPI clock activity, so these are only DC measurements.)

Since multiple known-good BIOS images produce exactly the same behaviour, I'm starting to believe this is no longer a BIOS issue.

Could someone point me to the next logical measurements for this board?

Should I check:
- RSMRST#
- PLT_RST#
- SLP_S3#
- SLP_S5#
- PCH power-good signals
- EC signals
or something else?

Any guidance would be greatly appreciated.

Thanks in advance
Michael

[mon2]

USB VBUS:
0V

Remove all power. Meter in resistance mode. What is the resistance to ground of the same VBUS line? Testing if the USB load switch is defective and causing this fault.

[Mize27]

Resistance from VBUS to GND with all power removed is OL (meter shows "1"). No short to ground. Meter probes together read ~0 Ω

[mon2]

Is your meter auto scaling or a manual type? If manual type, select a higher meter scale till you have a measurement.

Review if there is a usb load switch nearby the usb connector. Share the topside markings of this part.

[Mize27]

mon2

My meter is manual ranging. I measured VBUS to GND with the board unpowered. On the 20 kΩ range it reads OL ("1" on the display), so there is no short to ground, i also tried on 200K.

Please see the attached close ups of the USB area. I reckon you mean USB802 (AP5934 VW56F)?

[Mize27]

Found the EC: Microchip MEC1515-NP.

EC area measurements:
- 3.3V present near MEC1515 decoupling caps.
- Other caps around EC show 1.8V and 0V as expected.

Coils near EC:
PL5101: approx. 3.0V on one side, 1.2V on the other side
PL4501: approx. 2.7V / 3.0V

Could someone please point me to EC_RST#, RSMRST#, SLP_S5# or PLT_RST# test points for this board?

EDIT:
CPU VCore inductors: 0V on both sides.
CPU VRM MOSFETs have ~20V VIN.
Several small capacitors near the CPU VRM area measure 5.13V.
However, the CPU VCore never comes up.

The fan starts immediately when the AC adapter is connected, but there is no POST and no display.

At this point it looks like the CPU VRM is not being enabled. I'm trying to determine whether the NCP81266 is missing VCC, VR_ON/ENABLE, or another prerequisite power-good signal.

If anyone has a boardview/schematic or knows the VCC and EN pin/test points for the NCP81266, I'd really appreaciate it.

Mainboard DP/N: 03GNH6
Board markings:
SELEK MLK N18P
19795-1