Oxygen Sensors
Vehicles with OBD have an additional sensor (2) as well as the previously standard oxygen sensor (1) for mixture control. This second sensor is located downstream of the catalytic converter. Its task is to monitor the functioning of the catalytic converter and to give the driver a warning signal in the event of a fault.
The oxygen sensor (1) for monitoring the mixture operates in the same way as previously known oxygen sensors. The setpoint curve for a measurement using an oscilloscope has a sinusoidal shape with a frequency of between 0.2 and 2 Hz. If the frequency drops below 0.2 Hz, a trouble code is set and the exhaust gas telltale (MIL) is activated.
The OBD requires an additional check of the catalytic converter. This is performed by the additional oxygen sensor (2).
The oxygen sensor (2) monitors the ability of the catalytic converter to convert pollutants.
The catalytic converter is able to store oxygen in the lean phase. The stored oxygen is completely or partially consumed in the rich phase. The storage capacity of the catalytic converter is reduced by ageing and environmental factors. This increases the HC content in the exhaust gas. In accordance with the OBD requirements, HC emissions must not exceed a specified limit.
The reason for monitoring the catalytic converter is to provide information about the HC content in the exhaust gas based on the oxygen storage capacity of the catalytic converter. In this procedure, the oxygen storage capacity of the catalytic converter is determined from the signal of the oxygen sensor downstream from the catalytic converter with the aid of extensive evaluations. Certain vehicle conditions are a prerequisite for these evaluations. They can only start when the engine and catalytic converter have reached operating temperature. The vehicle speed for the test must be between 40 and 70 km/h. The test lasts for about one minute after the corresponding vehicle conditions have been reached and is performed repeatedly during one driving cycle.
The signal from the oxygen sensor (2) is heavily dependent on the fluctuating oxygen load (shortage/excess) of the catalytic converter. A measurement at this oxygen sensor (2) displays a different characteristic to one at the oxygen sensor (1) for mixture monitoring and can only be evaluated to a limited extent for services purposes.
The oxygen sensors are checked for short circuits, plausibility of the signal and frequency by the electronic control unit. The heating of oxygen sensors is only checked with regard to the correct resistance.
All relevant faults are indicated as MIL faults if they occur in three successive driving cycles.