To cleare the cache: activate developer option, unlock OEM, shutdown your phone, press volume up and power (to enter in Recovery / Fastboot mode), select "wipe cache partition", select "reboot system now" ... et voila.
I can't seem to get to the bootloader even with the OEM set to on within the Dev options. The only thing that appears is a message saying to release the power button with a 10 second countdown to switch off.
Putting the phone on charge doesn't help. I power off the phone. Hold power and up volume. The phone vibrates then a message ' please release key to power off'. The fastboot.... message did appear at the bottom of the screen after several attempts but nothing else happens.
Press power and volume up when the android and no command screen appears. Don't hold just press. The recover screen will appear. There is a graphics test option on it which tries to install an update. No idea if it does or not. Never seen that option in recover before.