The purpose of the OBD II Drive Cycle is to run all of the emission-related on-board diagnostics. When all diagnostics have been run, the System Status (Inspection/Maintenance ready) “flags” are set to “Yes.” And more about GM projects,check here:GM Repair Projects
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System Status flags must be set to “No” in the following cases:
* The battery or PCM is disconnected from the wiring harness (all flags must be set to “No”).
* The vehicle is new from the factory and has not been through an OBD II Drive Cycle (possibly all flags must be set to “No”).
* The PCM’s DTCs have been erased after completion of repairs. You can use the Service Programming Assistant to determine if on-board diagnostics is complete. To do this, go to ANALYZE on the main screen menu.
After the vehicle has been analyzed, go to the Inspection / Maintenance Learn Procedure and click the ‘Retrieve’ button to get the vehicle Inspection / Maintenance status. The system will tell you if the Inspection / Maintenance Set Procedure must be performed.
In this way, you can determine the status of the I/M flags. Remember I/M Status only reports on whether or not a diagnostic has been run-not what the test’s outcome was. In addition, for multiple sensor diagnostics such as oxygen sensors, tests must be completed for all sensors before the I/M flag will set.
The following is a list of the tests that are performed during the Drive Cycle and the requirements that must be met for the I/M ready flags to be set (refer to I/M drive cicles images in Advanced section).
A cold start is determine by measuring Engine Coolant Temperature and comparing it to the ambient air temperature during start-up. The PCM considers the engine to be cold if the following conditions are met:
-ECT less than 50 C (122 F)
-ECT and IAT are within 6 C (11 F) of each other at start-up
DO NOT leave key ON prior to cold start or HO2S heater diagnostic may not run.
2) IDLE (2 min., 30 sec.)
During this period, the O2 heater, “Passive” AIR, Purge “NO Flow”, Misfire, and possibly Fuel Trim (if closed loop is achieved) diagnostics will run. The A/C and electrical loads will help the Misfire diagnostic by stabilizing normal combustion variation.
Prior to accelerating, turn off the A/C. Then apply half throttle until 55 mph is achieved. During acceleration, the Misfire, Fuel Trim, and Purge flow diagnostics will run.
4) STEADY STATE CRUISE
During this portion of the cycle, the O2 “Response”, AIR “Intrusive”, EGR, Purge, Misfire, and Fuel Trim diagnostics run.
During the deceleration portion of the Drive Cycle, the EGR, Purge, and Fuel Trim diagnostics will run. It is important to have a gradual coast down with NO brake application. If the vehicle is equipped with a manual transmission, DO NOT depress the clutch, and remain in high gear.
Apply ¾ throttle until 55-60 mph is reached. The same diagnostics will run as during the last acceleration portion of the Drive Cycle.
7) STEADY STATE CRUISE
During this portion of the Drive cycle, the Catalyst Monitor diagnostic runs. Note that if the catalyst is marginal and if a battery disconnect has occurred prior to the Drive Cycle, it may take six separate drive Cycles to determine the state of the catalyst. The same diagnostics performed during the last steady state cruise repeat during this portion of the Drive Cycle.
The same diagnostics that were performed during the last deceleration portion of the Drive cycle are repeated again. Remember, NO clutch, NO brake, NO manual downshift.
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