You may be aware that it is illegal to use a Transponder to less than its maximum capability (i.e. in ALT mode for a Mode C or S transponder) unless requested to do so by ATC.

If you have a Mode S ES transponder with connected GPS correctly configured to broadcast ADS-B Out you may not be aware that if you deselect ALT mode then the transponder will NOT include your altitude in the ADS-B data stream.

That means any other pilots equipped with ADS-B In Electronic Conspicuity devices will NOT get traffic alerts if your flight path conflicts with theirs.

If you have a Mode C or Mode S transponder, set it to ALT mode when in flight (unless requested otherwise by ATC). It might help avoid a mid-air collision and save your life and the lives of others.