New Macbook Pro turns off with external monitor attached

[macbook123]

I have a new TB 15.4" Macbook Pro. It works with my 4k TV (Vizio) if that is attached via USB-C to HDMI, but it doesn't work in clamshell mode with my 4k Monitor (Dell) if that is attached via USB-C to DP: after a few seconds the DP signal is lost and the laptop seems to have gone to sleep. Is this something that could be modified via settings or do I need to send it back?

 

[MarkZ3]

Do you have the power plugged in? With the MacBook (12") it only works in clam mode with the power plugged in. Not sure if it changes anything for the 15".

 

[andy9l]

You need power alongside a connected keyboard and mouse in order for clamshell mode to operate.

https://support.apple.com/en-gb/HT201834

 

[macbook123]

Thanks, I'm embarrassed to admit that I did in fact not have power plugged in.