I built a new system last year and finished it with an 8GB Sapphire RX 480 (model 100406L). Everything ran smoothly, no issues, then I decided to buy another 480 and crossfire them. I purchased a 8GB XFX RX 480 (model RX-480P836BM) since their core clocks were pretty close. I installed the XFX card in the second slot, booted, and everything seemed fine although I didn't notice an increased FPS in fraps. shortly after this I started getting "Default Radeon WattMan settings have been restored due to unexpected system failure" and high temp errors and when I checked the cards the sapphire was reading fine but the XFX was reading 0rpm on the fans and some ungodly 511 celsius temp. I inspected the cards and neither were warm and the fans were running on both so I figured it was just a glitch.
It should also be noted that initially the settings auto selected crossfire but after that the crossfire option disappeared (though both were still shown in wattman)
I updated the drivers to 16.12, then 17.2 but to no avail. I have swapped the position of the cards in hopes that this would do something but no. I now have the XFX running solo and it is operating fine but if I add the sapphire I lose all display and have to shut down/pull it/reboot. I have wiped everything with DDU and all that jazz. I also fully admit that I am not super savvy when it comes to PC things so don't be afraid to explain things like a 10 year old or point out something dumb that I've done, heh.
CPU: i7 6700k
mobo: Asus z170 pro
ram: 16GB g skill ripjaws
cards: 8GB sapphire 480 and 8gb XFX 480 (spec images attached)