Identifying and replacing a worn-out O2 sensor can improve fuel economy, improve performance and driveability, potentially save the catalytic converter from damage and even failure, and dramatically reduce harmful emissions.

A study by Sierra Research Inc., an air pollution consulting firm, indicates half of vehicles that fail an HC or CO emissions test have one or more worn-out or inoperative O2 sensors. This study indicates that replacing all degraded oxygen sensors nationwide would reduce total hydrocarbon and carbon monoxide emissions more than all other automotive repairs combined.

one or two wire connectors, and the later heated sensors, with three or four wire connectors. Now two new, more sophisticated O2 sensors are beginning to appear in your bays. They provide faster, more accurate input readings for more efficient fuel management, and from here on in you will be seeing a lot of them.

Called "planar" sensors and "wide-band" sensors, they are at the same time similar and different from the older heated O2 sensors.

Light, Durable, Fast Warm-up

Heated planar sensors use a flat ceramic zirconia element, 1.5-mm thick, rather than the tubular ceramic 'thimble' used in previous O2 sensors. The electrodes, conductive layer of ceramic, insulation and heater are all laminated together on a single strip, which makes it smaller and lighter and more resistant to contamination. The heater element also uses less electricity and brings the sensor up to operating temperature (625-650 degrees Fahrenheit) in 10 seconds, which allows it to send accurate air/fuel ratio readings to the vehicle's engine management system while the vehicle is still warming up. This helps control cold-start emissions.

First used in the 1998 2.0 liter Volkswagen Beetle, the planar sensor is now being used in several domestic and import vehicles. They are ready to send signals faster and are more durable, with a solid ceramic sealant that retains the sensor element within the sensor casing and a dual-wall guard tube that protects the sensor element against excessive thermal or physical stress. Otherwise, these sensors function much like the earlier heated O2 sensor, sending higher or lower voltage signals - quickly - to the engine management system if the exhaust is either too rich or too lean.

Planar sensors use four wires running to a spade-type connector. One wire is the signal, one wire is ground, one wire is the heater element positive, and the fourth wire is heater element ground. Within the next few years, the planar sensor is expected to account for more than 50 percent of all new O2 sensor applications.

Infinitely Variable Readings

The heated wide-band sensor, installed mostly in imports, takes the planar sensor concept further, and adds a tiny "oxygen pump" to the planar strip to produce a "dual sensing element."

The wide-band sensor can actually measure the air/fuel ratio, and produces a variable signal directly proportional to the air/fuel ratio, rather than switching back and forth between rich or lean. This sensor sends a gradually changing signal, from very rich all the way to extremely lean, allowing the engine management system to add or subtract fuel as needed to maintain the optimum performance level.

The wide-band sensor uses five or more wires in a spade-type connector, rather than the three or four used by previous heated and heated planar sensors. One wire is the signal, one is signal wire ground, one wire is the heater element positive, one wire is for heater element ground. The fifth wire is for the "oxygen pump" signal wire output, and if it has a possible sixth wire, that is the "oxygen pump" input.

Planar sensors can be diagnosed for proper function with the same techniques used for earlier heated sensors. Wide-band sensors currently can only be diagnosed with a scan tool through the vehicle's on-board diagnostic system, and exact methods vary with each vehicle.

Make sure that you replace a heated planar sensor with a new planar sensor - if this type of sensor was originally installed and is called for by the vehicle manufacturer, the engine management system requires the rapid start-up and sensitivity of the planar sensor. And if you encounter an O2 sensor with five or more wires it is a wide-band sensor. Replace this wide-band sensor only with a new wide-band sensor, which is essential for proper operation of the engine management system.

Chuck Ruth is director of Product Management, Engine Management and Safety Products for Robert Bosch Corp. His product responsibilities include oxygen sensors, fuel injectors, ignition systems and other engine management products. Bosch, one of the world's largest suppliers of automotive sensors, developed the original oxygen sensor and continues to be an industry leader in sensor development.

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