Hi there,

I need your expert support!

Problem:

My laptop doesn't detect the external display anymore (though it worked before). Connected via Thunderbolt / USB C.

History/What I tried:

- The same issue happened before. Laptop was brand new. Worked for two weeks, then all of a sudden: black display. Contacted lenovo support. Made sure all driver / firmware updates are installed. Also tried all the OBVIOUS settings in windows (try to switch between internal display only to duplicate display etc. // windows+p). Afterwards (still not working) lenovo offered to exchange the laptop against a new one.

- The new identical one worked for three days with the screen before the external display again WENT black. Same procedure with the Lenovo support. They were out of ideas and sent me a third new laptop.

- While waiting for the new laptop, we bought a new display, too! In this configuration, it now worked for quite a few weeks - and now it's dead again. So the same issue happened with three identical laptops and two completely different screens (one old LG, one new Dell).

- Tried the displays with another laptop (this time via HDMI, not thunderbolt) - it works. Tried to connect my TV to the "problem-laptop": Black screen.

Suspicion:

I feel like this has something to do with power supply. With laptop #2 the screen went black in the exact moment I pulled to power cable and it went to battery mode. Afterwards it never came back (also when connected to the power). Also with laptop #3 I THINK it happened after a similar situation. Also one of the three identical laptops once showed a warning message that read something like: "can't display on external screen. not enough power".
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The laptop:

Lenovo Ideapad 720S

Windows 10

Display connected though Thunderbolt port > Adapter to HDMI > HDMI cable

So... any idea what's wrong here? Don't want to get a fourth laptop just to have the same issue after a few weeks again.

Thank you for your help.I wonder if the external display is drawing too much power and causing problems with the USB port in the laptop.  I think I would try getting a USB hub with it's own power supply and plugging the hub into the laptop then the external display into the hub.Welcome to the forum. I change the title of this thread. Moving forward, PLEASE use a subject that relates to the problem / question. Thanks.My guess is the use of the Thunderbolt port .
Do a search for:
 thunderbolt port vs usb c
... and find articles about possible issues.
And it is possible that item is bad.
One of many:

https://www.howtogeek.com/449991/thunderbolt-3-vs.-usb-c-whats-the-difference/ Quote

It’s not clear if Intel plans on updating Thunderbolt to version 4, but the future for Thunderbolt 3 is very clear. Intel’s Thunderbolt protocol is merging into USB4. The specification for USB4 was announced in the summer of 2019, with USB4 based products rolling out in 2020 or 2021.