On-Board Diagnosis System II - Volkswagen Technical Site

System Components (Gasoline Engine)
Was does OBD II test?
An aged or faulty catalytic converter has less oxygen absorption capacity. Therefore its conversion
capability is impaired. If the valid limit values for the concentration of hydrocarbons in the exhaust gas
during of a mandatory emissions test are exceeded by a factor of 1.5, this must be detected on-line.
Catalytic conversion diagnosis
During the diagnosis, the Motronic control unit
compares the voltages of the primary and
secondary catalytic converter probes. It determines
what is known as the primary to secondary
catalytic converter ratio (lambda probes I+II).
The engine management system interprets any
deviation from the nominal range as a malfunctioning
of the catalytic converter.
After meeting the fault conditions, the fault code
is saved to fault memory.
The fault is indicated by the self-diagnosis fault
warning lamp (MIL).
Catalytic converter
OK
Catalytic converter
NOK
175_041
175_042
U
U
U
U
t
t
t
t
Primary catalytic
converter
Secondary
catalytic converter
Primary catalytic
converter
Secondary
catalytic converter
V = voltage, t = time
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