Lenovo T480 NM-B501 no picture

[master3112]

I have a T480. After pressing the power button, the device boots up. I can hear the Windows welcome melody. However, the screen remains black.
The device has a CPU with an integrated GPU. If the device is switched on briefly without the CPU cooler, only the CPU core area heats up under the thermal camera. I measured the PU402 and PU403 inductors. These are responsible for the core voltage. I have 1.1V at these. Everything is OK, therefore the device starts up.
But I have 0V at the PU404 inductor, which is responsible for the GPU voltage. The power management of the CPU is solved using an NCP81218 controller and four ON302045 drivers from Onsemi.
PU402 VCCCPUCORE
PU403 VCCCPUCORE
PU404 VCCGFXCORE_I
PU405 VCCSA
I have measured the ON302045 drivers and compared the values. The only difference is the PWM voltage by which the driver is activated by the PU401 PWM-controller. I have 0.26V for the drivers that supply voltage. With the PU404, however, I have 2V at the PWM Pin. I also have a Lenovo X1 Extreme with exactly the same problem. The CPU power management is solved in the same way on this device. I also have 2V at the PWM-Pin of the driver for the GPU voltage on this device.
My question is, does anyone know whether it is possibly due to the 2V at the PWM PIN that the driver remains deactivated? That is my suspicion. If so, why is the driver disabled by the controller? I have the suspicion that the solder connections of the CPU are not in order and it needs to be reball the CPU? Does anyone have experience with such symptoms?
Thanks to the community

[FriedFred]

You measure those PWM voltages with a standard multimeter? A multimeter integrates any square-wave signals and the result is a voltage lower than 3.3V. So it would be interesting to check the PWM lines if there is really a square-wave signal.
The core voltage for the graphics module is usually 0.75V to 1.1V. If you do not have a dead-short at the "VCCGFXCORE_I" and you do not have a voltage either, you can feed-in a sufficient voltage by yourself and see what happens.
It's a difference if you measure really 0V or 20mV or 50mV. Real 0V mean there is not any activity, no voltage at all. But 20mV or 50mV mean there is a MOSFET working, a "LOW" signal.