Accessing boot menu when GRUB_TIMEOUT=0

[AAA BBB]

Dear GRUBbers,

how can I access the boot menu when GRUB_TIMEOUT=0?

I have searched over the net but none of the proposals I found worked.

For instance, here 
<https://unix.stackexchange.com/questions/219061/grub-boot-loader-set-timeout-0-how-to-get-the-boot-menu> 
it is suggested to press ESC or SHIFT after post, here 
<https://askubuntu.com/questions/262965/grub-timeout-set-to-0-cant-access-grub-menu-anymore> 
it is suggested to press SHIFT as well.

I asked because my system did not boot anymore and I had to boot into an 
emergency shell.

Thank you and best regards.

P

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