My garmin edge 305 powers down as soon as i press the mode butto

Contrary to the advertising, this unit is NOT waterproof. I booked mine in with Garmin for repair after taking a drink of saltwater whilst on the deck of my kayak. As soon as I advised them of the issue, they advised that it is not waterproof and offered me a very sharp price on a replacement.

I've just gotten back from a ride in extremely bad weather. I noticed when I was out the screen went blank, though if I pressed the key in the top left, the backlight would turn on.

After getting back home and having the unit dry out for a while, when powering on if I press the 'mode' key the unit instantly powers off. The 'up' 'enter' and 'down' keys are non responsive.

To resolve, I pressed the "start stop" key, and then the 'lap/reset' key, and then power the unit off. After doing this and powering back on again, the unit was completely functional once more.

I had the same problem after a rainstorm. Although the unit is supposed to be waterproof, it was not. Garmin replaced the unit when the mist did not clear after a few weeks, and the unit never worked after that.