The Dell XPS 13 Plus 9320's brightness control might not work due to a few reasons, including issues with the touchbar's ambient light sensing, driver problems, or BIOS settings. A simple workaround for the touchbar is to put the laptop to sleep and then wake it up, which can reset the brightness. Other potential solutions include updating drivers, checking BIOS settings, and verifying the Fn lock status. 
Here's a more detailed breakdown:
1. Touchbar Brightness Issues:
Problem:
The touchbar's brightness control might get stuck at full brightness, even in darker environments. 
Workaround:
Put the laptop to sleep and then wake it up. This can reset the touchbar's ambient light sensing. 
Troubleshooting:
If the problem persists, check if there are any BIOS updates available that address brightness control issues. Also, ensure that the latest firmware is installed for the touchbar. 
2. Driver and BIOS Problems:
Problem:
Outdated or corrupted graphics drivers or BIOS settings can affect brightness control. 
Solution:
Update drivers: Open Device Manager, go to Display adapters, right-click on your Intel graphics driver (e.g., Intel(R) UHD Graphics), and select "Update driver". 
Check BIOS: Access the BIOS settings during startup (usually by pressing F2 or Del) and look for brightness-related settings. Ensure they are configured correctly. 
Check for BIOS updates: Visit the Dell support website and download the latest BIOS version for your XPS 13 Plus 9320. 
3. Fn Lock and Function Keys:
Problem: The brightness control keys (usually Fn + F6 or Fn + F7) might not work if the Fn lock is enabled.
Solution: Toggle the Fn lock by pressing Fn + Esc. Try the brightness keys again. 
4. Automatic Brightness Adjustment:
Problem:
The automatic brightness adjustment feature might be interfering with manual brightness control.
Solution:
Go to Windows Settings > System > Display and disable "Change brightness automatically when lighting changes". 
5. General Troubleshooting:
Check Power Options: In Windows, go to Power Options and ensure that the power plan is not set to limit brightness. 
Restart your computer: Sometimes a simple restart can resolve minor glitches. 
Consider a clean install: If the problem persists, consider a clean installation of Windows. 

To receive assistance from Dell chat support, they need to verify the warranty status and ownership. Then you must troubleshoot with them.  Click the "Get Help Now" icon on the right to start a live chat session. If already out of warranty, click here for the Dell out of warranty offering.

@DELL-Jesse L​ 

I forgot to say one important point. Brightness control is working without any problems when my laptop is plugged in. But when it is powered by batteries there is no brightness control at all, brightness became on maximum :( What is a solution?