On the subject of spares, I highly recommend the practice of installing spares and keeping the previously working units as spares. This will assure that the spare fits and works. Nothing (almost nothing) is worse than having to find and install the spare only to find that it is defective, or doesn't fit, or needs adjustment/programming. This practice ensures that the spare worked when last installed. We vacuum seal all spares, when size permits. Bob Lowe MV Dreamer, presently in Mexico