Forcing a resolution or improving HDMI detection?

Hi guys

I built a raspberry pi box for my FIL, and as you might expect, they aren't very technically savvy.
They live an hour away, so it's a pain for everybody involved when things go wrong of course.

What's happening (my guess, mind you.. this hasn't happened in front of me) is that the TV does not detect the box.
And/or the box is defaulting to a resolution that the TV does not support. (Like 640x480, which I see happen on the box sometimes when I have it here).

Can anyone suggest to me how to force a resolution for all time on the box? Or improve the detection somehow?

thanks.

PS. Sorry, I know this isn't a lot to go on. Sometimes it works, sometimes it doesn't, but it always works for me And then it's fine for months.

Quote from lrusak

on RPi you can use config.txt

see, RPiconfig - eLinux.org

specifically,
hdmi_group=1
hdmi_mode=16

Hey thanks for responding.

I see the reason for hdmi_mode=16, but can you explain why hdmi_group 1 is better than hdmi_group 2?

thanks

Thanks gents, much appreciated. I've added those changes and hopefully it'll work when it gets back to them.