Your Mercury oxygen sensor can handle the constant usage and extreme heat of the exhaust system - but it cannot tolerate getting dirty! The oxygen sensor needs its surface to remain relatively clear because any substance on its exterior will interfere with its job of sending data to the engine's Electronic Control Unit. The data is send via voltage to the ECU. The voltage is created by the reaction of two layers within the sensor that create an electrical current. These two layers, made of platinum with another layer of zirconium dioxide between them, will create positive voltage when the oxygen becomes hot enough. One layer measures the air in your Mercury engine, while the other layer checks the oxygen levels in the exhaust. So, if any substance coats the ceramic exterior, the oxygen sensor can become sluggish and then finally stop. Using leaded gasoline in an unleaded fuel system will create havoc. In addition, certain sealing compounds for gaskets will coat the sensor, so take care to use 'sensor friendly' gasket compounds. Any other substances that can leak into the exhaust such as coolant and oil will interfere with the oxygen sensor's work. You might notice that the oxygen sensor is sluggish when the fuel exhaust emits too much excess fuel. That occurs when the ECU orders the fuel injectors to release too much fuel, creating a 'rich' mixture when it does not receive data from the oxygen sensor.