I have a small problem with my Trip computer on my Focus 2008...When I first start the car in the morning I have to press the side button to get how many miles of fuel I have left in the tank...So is there a way I can set my trip computer to show this option all the time?

Sounds like its not holding its memory , which ever option i select last , remains on the screen next time i start the car , perhaps a trip to the dealer if it's still in warranty.

It’s more annoying then anything else...My dad had a focus same year same model and his stays on how much fuel left in tank mine just says ‘Aux. Heater Off’

That is worrying, unless you have a bad chip, theres no reason that it should be losing power, especially as it remains in memory by taking some power from the battery....