Chapter 18 Diagnosis and Repair of the Emission Control and Exhaust Systems 381
Copyright by Goodheart-Willcox Co., Inc.
Hydrocarbons, oxides of nitrogen, and particulates are
measured in parts per million. The lower the parts per mil-
lion, the better the reading. Carbon monoxide, oxygen, and
carbon dioxide are measured in a percent of the total gas.
Carbon monoxide should be as low as possible, as this is an
indicator of air-fuel ratio and catalytic converter effi ciency.
Few states test for oxygen content, but this reading should
fall within a certain range. Some states set a minimum
requirement for carbon dioxide emissions. The actual per-
centage should be higher than the minimum requirement.
Typical tailpipe emissions readings for late model vehi-
cles with no defects are given in Figure 18-9. Always check
the latest published specifi cations for the engine and model
year of the vehicle being tested. Problems that can cause
excessive exhaust emissions are shown in Figure 18-10.
Comparing Exhaust Analyzer Readings
with State Inspection Readings
If the vehicle failed a state air-quality test, compare
the readings from the state test to your exhaust analysis,
Figure 18-11. The readings from the two tests will be simi-
lar if the problem that occurred during the state test was
present during your test. However, if the readings are dra-
matically different (the state reading shows only CO to be
high and your reading shows HC to be high, for example),
recalibrate the analyzer and retest the vehicle.
If the readings from the exhaust gas analysis you per-
formed are correct and you can fi nd no other problems,
road test the vehicle again and then conduct another
exhaust gas analysis. If the exhaust emissions remain cor-
rect, the engine was probably not at operating temperature
when the state test was done. If your shop is licensed to
reinspect failed vehicles for exhaust emissions, you may do
so at this time. Check local regulations before conducting
a reinspection.
Other Test Equipment
While the exhaust gas analyzer is a powerful tool for
diagnosing emissions problems, other tools are necessary
to locate which part is actually causing the problem. Earlier
in this chapter, you learned how to use vacuum and back-
pressure gauges to check the exhaust system for restric-
tions. Scan tools can be used to test electronic EGR valves
and to check the catalyst monitor on vehicles so equipped.
Other tools used to check emission control devices include
vacuum pumps, multimeters, and test lights.
Since many of emission controls are operated by the
ECM, precautions should be taken to avoid damage. Do
not use any test equipment on the computer-controlled
components unless the manufacturer specifi cally recom-
mends it.
Performing a Drive Cycle
The drive cycle consists of driving the vehicle for a
set time for specifi c acceleration, cruising, and decelera-
tion steps. It is performed whenever the battery has been
disconnected or before performing an emissions test. Some
states require that the drive cycle be performed before an
emissions test can begin. A scan tool is required to verify
the drive cycle has been completed and the appropriate
monitors set.
The drive cycle is different for each vehicle, so check
the service information. Carefully study the drive cycle
procedure and scan tool operation before starting. Be rea-
sonably sure that you can complete the drive cycle from
beginning to end. If the drive cycle has to be aborted for
any reason, the engine must be allowed to cool, which can
cause a considerable delay.
To begin the drive cycle test, check that the coolant
temperature is low enough to allow the ECM to start in open
loop. On most engines, the coolant temperature should be
Figure 18-9. Typical exhaust gas readings for a late model vehicle with no defects.
Tested Gas Standards
Unburned hydrocarbons (HC) 50 ppm
Carbon monoxide (CO) .5% (can also be measured in gpm)
Free oxygen (O2)
.5%; when CO and O2 are equal, the engine is running with maximum effi ciency for
lowest emissions
Oxides of nitrogen (NOX) 200 – 600 ppm (or .8 gpm)
Carbon dioxide (CO2) 12%; higher percentages indicate increased combustion effi ciency
Combined percentages of CO,
CO2, and O2
16%; variations between CO, CO2, and O2 are acceptable if the total is not greater
than 16%