Catalytic converter efficiency monitoring function - GF49.10-P-3033AMG

ENGINE 156.980 in MODEL 164, 251 

Fig 1: Identifying Catalytic Converter Efficiency Monitoring Function Diagram - Engine 156.980

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

By law, HC emissions must not go above certain limits. The task of monitoring catalytic converter effectiveness to obtain insights from the oxygen storage capacity of catalytic converters about their aging and therefore their HC conversion. The oxygen stored during the "lean phase" is then reduced totally or partially during the "rich phase". Aging leads to reduction in the storage capacity of catalytic converters whereby the HC conversion is removed.

When the firewall catalytic converter is operationally warm and lambda control enabled, the amplitude sizes of the O₂ sensor signals downstream and upstream of the firewall catalytic converters are compared. Changes in the oxygen content downstream of the firewall catalytic converters are almost completely dampened by the high oxygen storage capacity of the firewall catalytic converters.

This results in the O₂ sensor signal downstream of the firewall catalytic converters being virtually constant with a small amplitude. If the firewall catalytic converter is no longer operational, the O₂ sensor signal upstream and downstream of the firewall catalytic converters will be equal.

Wideband oxygen sensors are installed upstream of the firewall catalytic converter which can exactly measure the mixture composition. Downstream of the firewall catalytic converters or after the first catalytic converter inserts (for model 164, 216, 221, 251) there are voltage-jump oxygen sensors with which at 450 mV a sensor voltage λ=1 is recognized.

During the first stage, the oxygen stored when the mixture is rich (λ =0.95) is reduced until the sensor voltages downstream of the TWC are >650 mV.

In the next operation step, a lean mixture (λ=1.05) is set and the system waits until sensor voltages downstream of the TWC are <200 mV.

If the times so measured are under a stored characteristic the oxygen storage capacity is inadequate and the firewall catalytic converters must be replaced.

If a fault is detected, the engine diagnosis indicator lamp or CHECK-ENGINE in the instrument cluster lights up. The stored faults can be read out and deleted with STAR DIAGNOSIS.