Mercury - Quicksilver 8M2003959 Exhaust Oxygen Sensor MerCruiser EC Series Marine-Grade Construction 8M2003959. Overview. When you spend enough time on the water, you start to appreciate the small components that keep your boat running smoothly. The big, loud parts get all the glory, but it's the quiet sensors and seals behind the scenes that help your engine breathe, burn, and perform the way it should. The Quicksilver 8M2003959 Exhaust Oxygen Sensor is one of those unassuming pieces that gives