Why you need to service an oxygen sensor.

Since 1981, most light-duty gas engines found in road vehicles employed an oxygen sensor. By 1987, every spark ignition road engine was equipped with an oxygen sensor.

Due to more stringent emission control standards, they can now be found on off-road uses such as the farm’s UTV.

The terms oxygen sensor and lambda sensor are interchangeable.

Some modern Tier IV diesel engines do use an oxygen sensor, but it is for EGR control and not air/fuel ratio.

Every fuel has a stoichiometric value. It identifies the ratio of fuel to air for the most complete chemical exchange, or is simpler terms, the most efficient combustion.

For pure gasoline the stoichiometric value is nearly14.7:1 (fourteen parts of air to one part of fuel).

As the ratio goes numerically lower the mixture is richer: 12.7:1 is a richer mixture; there is less air with the same one-part of fuel. The converse applies.

Hybrid fuels such as E-10, E-15 and E-85 have a lower stoichiometric value since the combustion characteristics and energy content of alcohols are less than petroleum-based fuel.

When an engine is labeled as flex-fuel it may have an additional sensor in the fuel system that measures the ethanol content of the gasoline.

The engine controller alters the amount of fuel delivered to each cylinder via the injector to create the required stoichiometric value for that blend along with a modified ignition table.

Some flex-fuel engines use a complex algorithm that does not use an alcohol content sensor but relies fully on an advanced oxygen sensor, identified as a wide band.

A gasoline engine is often equipped with a catalytic converter (A catalyst is something that speeds up a chemical reaction without itself being consumed).

The purpose of the catalytic converter is to alter the exhaust gas from the engine into a benign form. This is called the conversion process.

For the catalyst to have a high rate of conversion two things need to happen: it must reach a minimum of 600 degrees F and the engine-out fuel mixture must be at or near stoichiometric. When this occurs, the catalyst is lit-off.

The unit looks like a muffler but contains various precious metals in a substrate that resembles a honeycomb.

There are numerous oxygen sensor designs based on the internal materials and other factors, but their purpose is the same. Their job is to act as an auditor and tell the engine controller if the mixture is at stoichiometric. If it is not, the fuel injection system either leans or richens the air/fuel ratio to satisfy the oxygen sensor.

The most used oxygen sensor produces a minute voltage of between 0.100 volt to 0.900 volt.

The voltage is created by the chemical reaction of the exhaust and the material in the sensor.

When the mixture is rich, the sensor output is high (above 0.450 volt), when lean it is low.

Stoichiometric is around the mid-point of the voltage range.  

The sensor has a port that samples the oxygen content in the atmosphere and measures that proportionally to what is found in the exhaust gas. That is how it determines the mixture strength.

The sensor is considered a consumable and needs to be replaced as part of your maintenance program. As the sensor ages and is exposed to exhaust, engine oil and particularly engine coolant from a head or intake manifold gasket failure, the output skews. It requires a richer mixture to produce the same voltage. Thus, it is telling the fuel injection system to add fuel when the engine does not need or want it.

At first the only telltale sign is an increase in fuel consumption. As the sensor further degrades and adds fuel it will result in diluting the engine oil with gasoline and washing oil from the cylinder walls.

Excessive wear to the engine bearings and piston rings will occur along with glazing of the cylinder wall.

A standard protocol in engine management is to allow twenty-five percent correction from the oxygen sensor.

That would mean the engine can be running at an 11:1 air/fuel ratio (extremely rich) and there will be no diagnostic codes in the system.

Once the 25% threshold is passed, the mixture is considered out-of-control and a trouble code would be stored. Many engines are ruined by a degraded oxygen sensor.

The sensor will last the longest when the engine is tuned properly, ingests no coolant or oil, and is not short duty-cycled. Extensive idling, numerous cold and warm restarts, along with engine coolant in the combustion chamber quickly degrades the sensor.

With most sensors costing less than $100.00, it is a wise practice to keep it fresh and accurate.