Snap-On Diagnostics DGA 1000 - Five Gas Combination Readings

7-4
Diagnostic Technique
Five Gas Combination Readings
This chart lists some of the possible combinations of exhaust gas
values and the most likely cause(s). Variables do not include
secondary air.
Legend: L = low; M = moderate; H = High
CO
CO
2
HC
O
2
NO
Possible Problems / Conditions
H
L
L
H
H
L
L
L
L
H
Thermostat or coolant sensor faulty. cold running engine
Thermostat or coolant sensor faulty - hot running engine
L L L H L Exhaust leak after catalytic converter
L H L H M Injector misfire, catalytic converter operating efficiently
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Injector misfire
Catalytic converter not working
Combination of rich mixture and vacuum leak
H
H
H
H
H
H
H
H
H
H
H
MH
H
L
L
L
L
L
L
L
L
L
L
L
L
L
LM
LM
LM
LM
LM
LM
LM
LM
LM
LM
LM
LM
H
L
L
L
L
L
L
L
L
L
L
L
L
H
L
L
L
L
L
L
L
L
L
L
L
H
L
Air-fuel ratio too rich:
rich fuel mixture
leaking fuel injectors
incorrect carburetor adjustment
power valve leaking
choke operating rich
float level too high
air filter dirty
EVAP canister purge system fault
PCV system problem
engine control computer malfunctioning
crankcase contaminated with raw fuel
all of the above with catalytic converter operating efficiently
rich mixture with ignition misfire
L
L
L
L
L
L
L
L
L
L
L
L
L
L
L
L
L
L
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Air-fuel ratio too lean
lean fuel mixture
ignition misfire
vacuum leaks/air leaks (between air flow sensor and throttle body)
EGR bad or vacuum hoses mis-routed
carburetor settings incorrect
fuel injector bad
O
2
sensor bad or failing
engine control computer malfunctioning
float level too low
LM
LM
L
L
LM
LM
L
L
LM
LM
Major engine fault;
compression low
camshaft lobe lift insufficient
LM
LM
LM
L
L
L
LM
LM
LM
L
L
L
H
H
MH
Ignition timing over advanced
Spark plug wire grounded or open
Engine control computer compensating for a vacuum leak
L H L L L Good combustion efficiency and catalytic converter