Volvo B6324S Functional Description page 9

The three-way catalytic converter (TWC) stores oxygen
found in the exhaust gases and uses it to make toxic
gases less dangerous. The catalytic converter is a TWC
converter in which hydrocarbons (HC) and carbon
monoxide (CO) are oxidized and oxides of nitrogen
(NOx) are reduced.
As the TWC ages its ability to store oxygen drops.
This reduces the conversion capacity of the TWC. To
avoid dangerous emissions the ECM checks TWC ef-
ficiency. This check is carried out as follows.
The two-sensor method stereo makes use of one
upstream and one downstream oxygen sensor for each
cylinder bank, each bank has one sensor before the
catalytic converter (UHEGO) and one after (HEGO).
Catalytic Converter Monitor Operation
DTCs
Monitor Strategy description
Typical catalytic converter diagnostic enable conditions
Enable condition
Ambient pressure
Vehicle speed
Catalyst temperature
Typical catalytic converter malfunction thresholds
Malfunction criteria
Accumulated signal fluctuation on
secondary O2 sensor during A/F
modulation (shift from lean to rich
/rich to lean)
Rich and lean lambda pulses are sent through the
TWC. For a TWC with good gas converter and large
oxygen storage capacity, it will take a long time for the
rich/lean pulse to reach the rear oxygen sensor. The
rear oxygen sensor will then have long rich and lean
pulses and a long time between switches. When the
TWC detoriate and oxygen storage capacity drops will
the rear oxygen sensor switching frequency increase.
The rear oxygen sensor voltage will be used to calcula-
te a test value of the TWC performance and a malfun-
ctioning TWC will be detected by OBD II
system.
P0420 - Catalyst System Efficiency Below
Threshold (Bank 1)
P0430 - Catalyst System Efficiency Below
Threshold (Bank 2)
High air flow monitoring
Minimum
74 kPa
0 km/h
550°C
Threshold value
Bank 1 > 22.5
Bank 2 > 22.5
Corresponding MonitorID
21
22
Maximum
655 km/h
1000°C
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