Catalytic Converter: Description and Operation

GF49.10-P-3033MOS Monitoring Catalytic Converter Efficiency, Function
ENGINE 275.953 in MODEL 216.376, 221.176 as of Model Year 09 / AEJ 08
ENGINE 275.982 in MODEL 216.379, 221.179 as of Model Year 09 / AEJ 08

Function requirements for monitoring catalytic converter efficiency, general points
^ Circuit 87M ON (engine control ON)
^ Engine running
^ Firewall catalytic converters at operating temperature
^ Lambda control enabled

Monitoring catalytic converter efficiency, general points
Hydrocarbon (HC) emissions should not exceed the limit specified by the legal requirements. The purpose of monitoring the catalytic converter efficiency is to evaluate the oxygen storage capacity of the firewall catalytic converters in order to obtain an indication of aging, and therefore of HC conversion.

For this purpose, the ME-SFI control unit (N3/10) reads the following sensors:

- LH and RH O2 sensors upstream of CAT (G3/3, G3/4), signals from oxygen sensor upstream of CAT (catalytic converter)
- LH and RH O2 sensors downstream of CAT (G3/5, G3/6), oxygen sensor signals downstream of CAT
- Crankshaft position sensor (L5), motor speed

The oxygen stored during the "lean operating phase" is then reduced totally or partially during the "rich operating phase". With aging, the oxygen storage capacity of firewall catalytic converter is reduced, and so therefore is HC conversion.
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.
Consequently, the signals from the oxygen sensors downstream of firewall catalytic converters have low amplitude and are virtually constant.

Monitoring catalytic converter efficiency, function sequence
When firewall catalytic converters are at operating temperature and the lambda control is enabled, the signal amplitudes of the oxygen sensors upstream of the firewall catalytic converters are compared with those downstream.

If the firewall catalytic converters are no longer operational, the oxygen sensor signals from the O2 sensors upstream and from the O2 sensors downstream of the CAT are the same size.

A number of measurements take place in the lower partial-load range in the specified engine rpm range.
The results are compared with a performance map in the ME-SFI control unit.

Any faults detected are stored in the fault memory of the ME-SFI control unit.
These can be read out and deleted with the Diagnosis Assistance System (DAS).

If a fault is detected, the ME-SFI control unit actuates the engine diagnosis indicator lamp (A1e58) on the instrument cluster (A1) through the chassis CAN (CAN E) and central CAN (CAN F).