Auto Service World
Feature   February 1, 2006   by Auto Service World

Countertalk: Knowledge Building: Oxygen Sensors

There once was a time when something called a carburettor was the sole determinant of the air/fuel ratio in the internal combustion engine. No kidding. Ask the most senior member of your staff. He has probably even seen one.

Today, we all have to deal with a very different set of components.

Among these, oxygen sensors (O2 sensors) are among the most critical components in the fuel delivery and emission control system. Properly operating O2 sensors tell the ECU when the air/fuel ratio is rich or lean, and the ECU adjusts the amount of fuel injected to compensate (or correct) the air/fuel ratio.

All O2 sensors use a zirconia ceramic element with platinum electrodes to create the rich (>800mv) or lean (<200mv) signal. The ceramic sensing element is packaged into a stainless steel body for protection and to allow solid mounting in the exhaust system. Every O2 sensor also requires “reference air” inside of the zirconia element to allow it to properly measure the exhaust gas.

There are several different types of O2 sensors.


One- and two-wire sensors are unheated sensors which rely on hot exhaust gases to warm up the zirconia element to its activation temperature. These sensor types have free flowing single layer tip shields to allow more exhaust flow into the element.


Three- and four-wire sensors are heated sensors which have resistor-based heaters to warm and keep the sensor element at its operating temperature. By heating the sensor, combustion control can become fully operational more quickly than if exhaust gases alone are relied on to bring the sensor to its operating temperature, as in one- and two-wire types.

Heated sensors are designed with restricted flow dual layer tip shields that prevent cooling of the element by exhaust flow during engine warm-up, and also protect the hot element tip from being hit by water droplets in the exhaust pipe.


One- and three-wire sensors use case or chassis grounding for the sensor return path. Two- and four-wire sensors have a dedicated sensor ground (return) wire and are often referred to as isolated ground sensors.

Body Type

Some O2 sensors were designed with laser welded “waterproof” housings and thereby seal the reference air inside. More advanced sensors use housings designed with a breathable membrane to keep water and contamination out and let fresh air into the element.

Certainly, diagnosis of O2 sensors and emission system problems has become more difficult as the complexity of the systems has increased, but there are typical failure modes or problems that are commonly identified. For example, O2 sensor element electrodes can deteriorate due to exhaust erosion or from contaminates that attack or block the electrodes from the exhaust gas. These conditions will cause the sensor to output a weak signal so that the rich and lean voltages will not be achieved. That means the rich voltage will be less than 800mv and the lean voltage will be greater than 200mv.

In fact, bad/poisoned oxygen sensors are the leading cause of excess harmful exhaust emissions, contributing to the greenhouse effect, according to a California Air Resources Board study.

Common causes of O2 sensor poisoning are lead or lead additives or octane booster used in the gasoline, ethylene glycol (anti-freeze/coolant) leaking into the engine, and some of the “oil enhancing” additives that allow high levels of silicone or phosphorus into the engine. Of course, any engine that is burning or otherwise consuming oil can also foul the O2 sensors.

Element poisoning is more prevalent in the unheated sensor types for a couple of reasons: One, the flow of exhaust to the element is more open, allowing more contamination to get to the element than in heated sensors; and two, the higher temperature of the heated elements actually prevents the poisons from attaching themselves to the element. This is sometimes referred to as “burn-off,” though this term does not really accurately describe the process.

Another typical failure point for the non-breathable / welded case O2 sensors is that the reference air that is sealed inside the housing can become contaminated by exhaust leakage (or blow-by) inside the sensor. This is known as air reference contamination and leads to reduced voltages during operation, to the point that everything reads lean to the ECU so it constantly commands a rich adjustment.

This condition will cause the SES light to come on after several minutes of operation and also causes poor idle quality and reduced fuel economy. This problem is prevalent in some of the major names in O2 sensors, so much so that some vehicle manufacturers have even tried to compensate for this failure in the vehicles’ ECU software.

One of the more common failure modes found in O2 sensor diagnosis is sensor or vehicle wiring harness damage.

Any damage to the wire coating or the connector seals can cause poor O2 sensor performance that looks like a bad sensor.

Water inside the connector will corrode the terminals and create a turquoise colour on the terminals or inside the connector. Field data shows that virtually every O2 sensor diagnostic code can be set by water in the harness connector. Technicians should always verify the vehicle wiring harness is in good condition before concluding that the O2 sensor has failed or become poisoned. Harness repairs must be made with waterproofed connections like epoxy-filled heat shrinkable tubing.

Another electrical failure point to be checked is the connection to chassis ground on a one- or three-wire O2 sensor; a good, low resistance connection of the housing to electrical ground is required for proper performance.

Broken ceramic elements in the sensor are easily identified by the rattle they create; however a cracked heater is most often found by checking heater resistance at the sensor harness connector. Technicians should probe the heater terminals and read the resistance: typical heater resistances are between 3 ohms and 10 ohms, so anything in that range is likely okay (check manufacturer’s data to be sure).

A cracked heater will be open or infinite resistance. Manufacturing strong, high quality ceramics requires considerable ceramic processing experience.

Repairing vehicles with O2 sensor or emission system problems requires high quality parts and a full understanding of the vehicle Engine Management Systems.

Many consumers may mistakenly believe that an O2 sensor trouble code generated on a scan tool means the O2 sensor needs replacing and that this will fix the problem.

This can be the case, but not always, and consumers and technicians should be well informed of the difficulty in diagnosing O2 sensor problems and the fact that O2 sensor failures may point to larger issues under the hood.

Special thanks to Delphi Product & Service Solutions, Robert Bosch, and NGK Spark Plug Canada for their contributions to this article.

Wide Band Sensors

Wide-band heated oxygen sensors expand on the planar design of most four-wire sensors by actually measuring the air/fuel ratio.

Instead of switching back and forth between two voltage outputs like conventional sensors, the wide-band O2 sensor detects a wide range of air-to-fuel ratios and produces an output signal directly proportional to the air/fuel ratio.

This advanced sensor is free of reference air, has a much higher accuracy, and offers precise and tighter control within the stoichometric point for lean-burn applications. This class of sensor also operates at approximately 700 to 800 degrees C, or about twice as hot as conventional O2 sensors. They operate differently, too.

Wide-band sensors combine the oxygen-sensing cell from the planar sensor with an oxygen pump to create a device that can actually measure air/fuel ratios.

The sensor’s core still senses oxygen in the same way that a conventional thimble-type O2 sensor does. To get the added precision, an oxygen pump uses a heated cathode and anode to pull some oxygen from the exhaust into a diffusion gap between the two components, and seeks to balance this oxygen level to produce precise stoichiometric 14.7:1 air/fuel ratio. The amount of current required to maintain this balance is directly proportional to the oxygen level in the exhaust, which gives the ECU the precise air/fuel measurements it needs.

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