Fast charge no longer works galaxy s8

[Starscream]

Fast charge no longer working. Stays on for 5 seconds the phone switches to cable charge. Am using cable and charger that came in the box.

[BandOfBrothers]

If this was my situation id try as a process of elimination...

Turn Fast Charging Off and On in the phone and Soft Reboot the phone by turning the device Off and On. 

Try another brick and cable with the phone. 

Try your brick and cable with another phone if you can. 

Try a fast charging desktop stand or pad to see if it fast charges that way. 

If no joy a Samsung Experience Store or Samsung Service Centre location can run a diagnostic on the phone.