Lenovo Thinkpad X1 Yoga 4th Gen (20QF-CT01WW) Fan Spin but No POST

[left_cap]

Hey Folks,

I have a 4th gen X1 yoga I'm trying to repair, current state:

- Laptop charges battery fine and or no obvious power circuit faults
- When power button is pressed, keyboard briefly lights up, power button lights up and stays on, brief fan spin
- No display or back light, does not respond to external display, and caps lock does not work (no response to keyboard presses)
- If device is left on the CPU fan will ramp up/down as the CPU warms up

To me it seems the device fails to POST, I have tried flashing a 20QF bios from the bios request page, and even flashed new thunderbolt firmware to its ROM as this device is part of the faulty thunderbolt firmware list. After flashing the device has the same symptoms as above.

[mcplslg123]

DRAMRST# present?

[left_cap]

DRAMRST# present?

I had a look at the schematic and board view:




From the above I don't believe "BU31" connects to any test pads on the mobo besides a pad directly under the BGA, and no reference to dram reset on the memory schematics... hopefully I'm just reading the schematic incorrectly but I think I'm SOL to test for dram reset.

[mcplslg123]

Ram is reset by cpu and going by the schematic, there is no component inbetween. So simply cant be tested.

Link the schematic/boardview for better response.

Ram is onboard soldered or slot type? If later check the consumption on dc supply
01.With ram
02.Without ram

[Nebukadnerzar_V]

I also have the 3 machines of this type with this exact issue.
Power / Charging seems fine, can be switched on and off with power button, but no picture (internal/HDMI), no reaction to keyboard, no beeps.
Reflashing BIOS (with clean one) does not change anything.
Also I noticed, the built in screen flashed for a split second - probably just backlight.

Ram is soldered, so nothing to do there.

Was there any progress with your machine, left_cap?

[left_cap]

I also have the 3 machines of this type with this exact issue.
Power / Charging seems fine, can be switched on and off with power button, but no picture (internal/HDMI), no reaction to keyboard, no beeps.
Reflashing BIOS (with clean one) does not change anything.
Also I noticed, the built in screen flashed for a split second - probably just backlight.

Ram is soldered, so nothing to do there.

Was there any progress with your machine, left_cap?

Yeah it seems I got lucky today as it's fully working right now... unfortunately I have no real idea why it decided to boot today of all days versus back when I made this post, but I'll do my best to retrace my steps for you along with my observations.

I first attempted to try alternate BIOS's from badcaps, including one an individual customised for me, alas it actually made the problem worse:

- No fan spin
- no keyboard light
- brief light on power button, then off
- no backlight

After a couple attempts I simply reverted to my OG BIOS, at this point I focused to see if I could find probe points for the RAM modules, but at least for my revision there appears to be no exposed test points, pads or leads I could use.

The last thing I tried was to extract a new ROM for the thunderbolt controller via a BIOS upgrade package, as I saw it mentioned it could be interfering with boot up, unfortunately this seemingly did nothing:

- Power button light
- Brief keyboard light activity
- Fan spin
- No display

At this point I kinda just let the laptop sit on the "healing bench" in pieces for a few months, as I was done trying to fix it. Today however I put everything back together and for the shits and giggles tried powering it on one more time fully assembled... to my disbelief the healing bench worked!

The only thing that changed in the time I was tinkering with the device, and left it for a few months in pieces was both the primary lithium battery and the backup BIOS settings battery remained unplugged from the motherboard for that period of time.

So my recommendation to you is simply unplug the main lithium battery and the backup BIOS settings battery from the motherboard, and give it about a week or so... Something tells me that one of the onboard controllers was halting the boot process in an error state; but removing all power sources eventually caused things to reset back to normal.

At this point I have ran the full set of UEFI BIOS tests (ie CPU, Memory, Battery, Display, Storage, HID etc) and every test came back as a pass, so no known hardware faults. I hope this can help you fix your X1's, just wish I could provide a more complete answer as to why mine works now, but my "solution" is just a theory as it's only one device.

[Nebukadnerzar_V]

Thank you for the detailed response!

Then I will at least not give up hope on my machines either

Could you provide the Thunderbolt Controller ROM and maybe some hints as to where to flash it to?
Even though it did not immediately fix your issue, I would still also like to give it a try on my machines.