Technical Topics No. 6

HOW DOES THE OXYGEN SENSOR WORK ?

This is the only sensor that makes its own voltage. The voltage signal is proportional to the amount of unburned oxygen in the exhaust. When hot (at least 600 degrees F), the zirconium dioxide element in the sensor's tip produces a voltage signal that varies according to the difference in oxygen content between exhaust and outside air.

The higher the concentration of unburned oxygen in the exhaust, the lower the differential across the sensor tip and the lower the sensor's voltage output. The sensors output ranges from 0.1 volts (lean) to 0.9 volts (rich). A perfectly balanced ("stoichiometric") fuel mixture of 14.7:1 gives a reading of around 0.5 volts.

Some 02 sensors have three wires and an internal heating element to help the sensor reach operating temperature more quickly. The heater also keeps the sensor from cooling off when the engine is idling.

An 02 sensors normal life span may be as low as about 30,000 to 50,000 miles (50,000 to 80,000 Kms). Sensors can fail prematurely if they become clogged with carbon, or are contaminated by lead from leaded gasoline or solvents from the wrong type of RTV silicone sealer.

As the sensor ages, it becomes sluggish. When the signal starts to lag behind changes in the exhaust, or becomes static, the engine experiences driveability problems (loss of power, rough idle, poor fuel mileage, or excessive emissions).

Sensor accuracy can also be affected by air leaks in the intake or exhaust manifold, or even a fouled spark plug. A misfiring plug allows unburned oxygen to pass through into the exhaust, causing the 02 sensor to give a false lean indication.

Most after market replacement oxygen sensors are of a "universal" design which means some wire splicing may be necessary during installation. Graphite anti-seize compound should be used on sensor threads unless they are precoated. The rubber boot that fits over the sensor should not be pushed down further than half an inch from the sensor's base.

Some vehicles are equipped with a different type of 02 sensor that has a titania rather than a zirconia element. Instead of generating its own voltage signal, a titania 02 sensor changes resistance as the air fuel ratio goes from rich to lean. Instead of a gradual change, it switches from low resistance (less than 1,000 ohms) when the mixture is rich to high resistance (over 20,000 ohms) when the mixture is lean.

The engine computer supplies a base reference voltage of approximately one volt to the titania 02 sensor, and then reads the voltage flowing through the sensor to monitor the air/fuel ratio.

When the fuel mixture is rich, resistance in a titania sensor will be low so the voltage signal will be high (close to 1.0 volt). When the fuel mixture is lean, resistance increases and the voltage signal drops down to about 0.1 volt.

Compared to the more common zirconia 02 sensors, titania sensors have three advantages: (1) they don't need an air reference (there is no internal venting to the outside atmosphere to plug up); (2) they have a fast warm-up time (about 15 seconds); and (3) they work at lower exhaust temperatures (they won't cool off at idle and they can be located further downstream from the engine or used with turbochargers).