Oxygen Sensor After Catalytic Converter
Oxygen Sensor After Catalytic Converter
Oxygen sensor after catalytic converter
The monitoring sensor with an erratic characteristic curve is a discrete-level sensor manufactured by Bosch, type LSF 4.2. This monitoring sensor is used after the catalytic converter for catalytic converter diagnosis. The monitoring sensor reliably identifies deviations from lambda = 1, but it is not equipped to determine the size of the mixture deviation.
Functional description
The front oxygen sensor measures residual oxygen in the exhaust gas. The fluctuating values of the residual oxygen are forwarded to the engine control unit as a current signal. The DME corrects the mixture composition by means of fuel injection. A second oxygen sensor (monitoring sensor) is built in behind the catalytic converter. The catalytic converter has a high oxygen storage capacity. This means there is only a little oxygen behind the catalytic converter. The monitoring sensor supplies a virtually constant (attenuated) voltage. With increasing age, the oxygen storage capacity of the catalytic converter declines. The monitoring sensor then reacts increasingly to oxygen sensor deviation with voltage fluctuations. These characteristics are used by a special diagnostic function for catalytic converter monitoring. A malfunction of the catalytic converter is indicated by the emissions warning light.
As residual oxygen remains in the exhaust gas during operation with rich mixture, an electrical voltage is present between the outer electrode and the inner electrode. To ensure that the reference size always remains the same, the reference air duct is connected to the ambient air. The reference size is therefore the residual oxygen content of the ambient air.
Structure and inner electrical connection
A protective layer is applied to prevent damage to the outer electrode due to residue in the exhaust gas. The ceramic layer, made from zirconium dioxide (ZrO2), is conductive to oxygen ions from approx. 350 °C. To ensure that the oxygen sensor reaches the operating temperature as quickly as possible, it includes an integrated heating element.
Signal shape and setpoint values
Thanks to a residual oxygen content that equates to a mixture composition of lambda = 1, the voltage measurement on the measuring cell indicates a voltage of 450 millivolts. When lambda = 1, the composition of the exhaust gas of the different material is ideal.
Observe the following setpoint values for the monitoring sensor:
Size Value
Lower voltage range of oxygen1 10.7 to 16.5 V
sensor heating
Voltage of oxygen sensor heating 4 V
in warm-up phase
Operating temperature ≥ 350 °C
Heating resistance at 20 °C 9 Ohms ±2 Ohms
Maximum pump current 2.2 to 2.6 Ohms
Diagnosis instructions
Failure of the component
The following Behavior can be expected if the monitoring sensor malfunctions:
- Fault entry in the engine control unit
- The emissions warning light lights up in the instrument panel
General notes
The following diagnosis monitoring functions check the condition of the engine and exhaust emission system:
- Oxygen sensor adaptation
The purpose of oxygen sensor adaptation (mixture adaptation) is to compensate for component tolerances and ageing effects affecting the fuel-air mixture.
- Catalytic converter diagnosis
The diagnosis tests the oxygen storage capacity of the catalytic converter. The oxygen storage capacity provides an indication of the catalytic converter's conversion capability.
IMPORTANT: Protect plug connection against dirt contamination
Ambient air is required in the interior of the monitoring sensor. The ambient air enters the interior via the plug connection through the cable. This is why the plug connection must be protected against soiling, e.g. by wax or sealant. In the event of faults in the lambda control, the plug connection on the broadband oxygen sensor must be checked for soiling.
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