it is sometimes simplistically described as a binary indicator, reading either "rich" or "lean". Titania sensors are more expensive than zirconia sensors, but they also respond faster. In automotive applications the titania sensor, unlike the zirconia sensor, does not require a reference sample of atmospheric air to operate properly. This makes the sensor assembly easier to design against water contamination. While most automotive sensors are submersible, zirconia-based sensors require a very small supply of reference air from the atmosphere. In theory, the sensor wire harness and connector are sealed. Air that leaches through the wire harness to the sensor is assumed to come from an open point in the harness – usually the ECU, which is housed in an enclosed space like the trunk or vehicle interior. Location of the probe in a system The probe is typically screwed into a threaded hole in the exhaust system, located after the branch manifold of the exhaust system combines and before the catalytic converter. New vehicles are required to have a sensor before and after the exhaust catalyst to meet U.S. regulations requiring that all emissions components be monitored for failure. Pre- and post-catalyst signals are monitored to determine catalyst efficiency, and if the converter is not performing as expected, an alert gets reported to the user through on-board diagnostics systems by, for example, lighting up an indicator in the vehicle's dashboard. Additionally, some catalyst systems require brief cycles of lean (oxygen-containing) gas to load the catalyst and promote additional oxidation reduction of undesirable exhaust components. Sensor surveillance The air–fuel ratio and naturally, the status of the sensor, can be monitored by means of using an air–fuel ratio meter that displays the output voltage of the sensor. Sensor failures Normally, the lifetime of an unheated sensor is about 30,000 to 50,000 miles (50,000 to 80,000 km). Heated sensor lifetime is typically 100,000 miles (160,000 km). Failure of an unheated sensor is usually caused by the buildup of soot on the ceramic element, which lengthens its response time and may cause total loss of ability to sense oxygen. For heated sensors, normal deposits are burned off during operation, and failure occurs due to catalyst depletion. The probe then tends to report lean mixture, the ECU enriches the mixture, the exhaust gets rich with carbon monoxide and hydrocarbons, and the fuel economy worsens. Leaded gasoline contaminates the oxygen sensors and catalytic converters. Most oxygen