Mercury - Warning Horn Kit Piezo Alarm Upgrade 816492A9. Overview. A warning horn is one of the simplest but most important pieces of onboard protection. The Mercury 816492A9 Warning Horn Kit gives your engine a clear, reliable audible alert when something needs attention-oil, temperature, or other key engine parameters. This kit uses the newer piezo-style horn, designed to work with later-model remote controls and harnesses, and brings modern reliability to engines that need a proper SmartCraft