User's Manual

Gas Analysis
71
Catalyst Systems
Where AFR is used for Non-Cat systems, Lambda is used for Cat systems and the
engine management systems control Lambda to 1. Any value >1 is a lean mixture and
anything <1 is a rich mixture.
Catalyst System Gases with Lambda
The catalytic converter converts CO and HC into water (H
2
O) and extra CO
2
. Cats
increase the CO
2
level, therefore they are useful for reducing acid rain rather than the
ozone layer. With a good operating system it should be possible to achieve 0 for both
CO and HC under normal operating conditions.
Fault Diagnosis
The following tables show various relationships between gases and suggest a
possible cause for these gas levels. H = High, VH = Very High, L = Low, VL = Very
Low, N = Normal.
5 Gas Relationship
Lambda 1
Gas Perfect Minimum Maximum Problem
CO In Specification 0.5
CO
2
Highest possible 12.5% 15 Before combustion
HC Lowest possible 50ppm During combustion
O
2
Lowest possible 2% After combustion
HC O
2
CO CO
2
NOx Possible cause
H VH Vacuum leak
H H VL Overactive EGR valve
H L H L L Running rich
H H H L H Running lean
H Increased combustion pressure or VH O
2
after
combustion
VH N VL VL Zero Blown head gasket
HHHL H Faulty CAT