This useful bit of info is taken from this node with the same name, on Everything2.

enabling diagnostics mode on a garmin eTrex GPS (thing) by gambit32 ? Sun Jan 14 2001 at 02:04 utc

Unfortunately, the method described at " enabling diagnostics mode on a garmin gps" doesn't work for the Garmin eTrex GPS. To enable diagnostic mode on an eTrex, Hold Up and Page when powering the unit on. You'll get a screen with a lot of information (http://home.wish.net/~marx/etrexd.jpg) Displays

BRAVO Ver 6            -Operating system version
Software ver. 2.09     -Software version
10:41:09 pm            -The time (f course)
temp         27'c      -Internal temp
bat          2.94v     -Battery voltage
ext--------------v     -External voltage(from outside source)????
freq 16368323          -The frequency it's tuned to?????
Drft--------------     -Frequency drift???????
Rdrj--------------     -Rejected? Rejected what?
Sgnl        1671       -Signal strength?  It keeps changing.
SNR            0       -Signal to Noise Ratio? It sometimes changes.
BRV          pass      -Something that was checked passed, but what?
RAM          pass      -Random Access Memory check passes?
ROM          pass      -Read Only Memory check passed?
^      70      ()      -70 what? has test buttons on both sides.
^              ()      -More button test icons.
() 994BC6DC            - ??????????