Chapter 18 Diagnosis and Repair of the Emission Control and Exhaust Systems 383
Copyright by Goodheart-Willcox Co., Inc.
below 120°F (50°C). Always check the service information
for the exact temperature at which the ECM will enter open
loop. Be sure to allow for variations in the coolant tempera-
ture sensor calibration. In warm climates, the cool down
period can take as long as six hours. Ideally, the vehicle
should sit overnight before beginning the drive cycle.
After ensuring that the engine temperature is low
enough to start the drive cycle, attach the scan tool. Set the
scan tool to record the ECM status as the engine operates.
Some scan tools have a dedicated drive cycle option.
A typical drive cycle will cover engine warm-up,
idling, accelerating, decelerating, and cruising in a specifi c
order, as outlined in the service information, Figure 18-12.
This order must be exactly followed. Some scan tools will
prompt the technician throughout the drive cycle.
Start the vehicle and complete the drive cycle sequence
as outlined in the service information. A typical drive cycle
takes from 8–15 minutes to complete, depending on the
manufacturer. Some drive cycles require the technician
to turn the air conditioning on and off at certain times or
decelerate with the manual transmission/transaxle clutch
engaged or released, depending on the portion of the cycle
being performed.
Performing the drive cycle with the vehicle on a chassis
dynamometer will allow the scan tool to gather readings in
the shortest-possible time. If the vehicle is driven on the road,
it may be impossible to complete the drive cycle exactly as
designed. For example, the drive cycle may require several
acceleration and deceleration sequences, followed by driv-
ing on a road where 55 mph (88 km) can be maintained
for several minutes. Fortunately, most scan tools take these
factors into account and allow the cycle to continue once
the proper conditions have been reestablished. Some scan
tools can be paused when the drive cycle must be delayed.
However, if the engine is turned off for any reason, the drive
cycle must be restarted.
The scan tool will indicate when the drive cycle is
complete. Most scan tools will only indicate when the
cycle is complete, not whether the vehicle passed or failed.
If any malfunctions were detected during the drive cycle,
they will be stored as trouble codes in the ECM and read
by the scan tool. Check for any stored trouble codes and
make further diagnostic checks and repairs as needed. After
repairs are complete, repeat the drive cycle to ensure that
the vehicle is repaired. Do not attempt to perform an IM240
test until the drive cycle is successfully completed.
Low Carbon Dioxide (CO2) Reading
Overview
A low CO
2
reading is usually caused by a rich air-fuel mixture or dilution of the exhaust gas sample.
Check all of the systems mentioned above along with the systems listed here.
Possible
Problems
• Exhaust system leak
• Defective input sensor
• Defective ECM
• Sticking or leaking fuel injector
• Higher-than-normal fuel pressure
• Leaking fuel pressure regulator
Low Oxygen (O
2
) Reading
Overview
Low O2 readings are usually caused by a lack of air or a rich air-fuel mixture—the same conditions that
can create excessive CO readings. Note: not all analyzers check the exhaust gas for O2 content.
Possible
Problems
• Plugged air fi lter
• Engine carbon-loaded
• Defective input sensor
• Defective ECM
• Sticking or leaking fuel injector
• Higher-than-normal fuel pressure
• Leaking fuel pressure regulator
• Oxygen sensor contaminated or responding
to an artifi cially lean condition
• Evaporative emission system valve defective
High Oxygen (O
2
) Reading
Overview
A high O2 reading is an indication of a lean air-fuel mixture, dilution of the air-fuel mixture, or dilution of
the exhaust gas sample by outside air. When a high O2 reading is present, the CO reading is usually
very low or does not register.
Possible
Problems
• Vacuum leak
• Low fuel pressure
• Defective input sensor
• Exhaust system lean near the tailpipe
Figure 18-10. These are the most common causes of high emissions readings. (continued)