Hi John:WILLIAM,a 15 code indicates a loose or poorly connected #4 injector wire,open or short in #4 injector or a failed #4 injector. generally the engine will not start. #4 should be the middle injector on the left.I got these codes afterdoing a search for F1 codeson the technical forum at http://www.gl1800riders.com . the true GL1800 guru here is Kit Carson. he should be around soon to answer any questions. If not PM him.I don't have a shop manual yet so my knowledge is limited to what I find in the forums.
hope this helps john