Circuit Description
The On-Board Diagnostic (OBD) System Check is an organized approach to identifying a problem created by a control module system malfunction. The OBD is the starting point for any driveability concern diagnosis. The system check directs the service technician to the next logical step in diagnosing the concern.
Understanding the table properly reduces the diagnostic time. Proper use of the OBD helps prevent the unnecessary replacement of good parts.
Diagnostic Aids
Important: Do not clear the DTCs unless directed by a diagnostic procedure. Clearing the DTCs will also clear valuable Freeze Frame and Failure Records data.
Inspect all related wiring and connections including the connections at the VCM. These may cause an intermittent .
Any base engine problem could alter the scan tool data. Examples of these are: distributor timing, valve timing, intake gasket leaks, etc.
An intermittent may be caused by a poor connection, rubbed through wire insulation, or a broken wire inside the insulation.
Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis in Wiring Systems.
If a repair is necessary, then refer to Wiring Repairs or Connector Repairs in Wiring Systems.
Test Description
The numbers below refer to the step numbers in the diagnostic table.
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This step determines if the scan tool powers up when connected to the DLC.
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The MIL should be ON when you turn ON the ignition.
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Refer to Diagnostic Trouble Code (DTC) List for a list of valid DTCs.
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Refer to Data Link Connector Schematics in the Data Link Communications in order to determine all of the modules communicating on the Class 2 Serial Data circuit. If any of the modules are not present in the Diagnostic Circuit Check, then the modules never established communications. The scan tool will indicate a YES state for communicating modules and a NO state for modules that had established yet no longer communicate.
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This test determines if the VCM can control the MIL.
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Comparing the ACTUAL and TYPICAL scan tool data values will aid in determining whether an engine parameter is within limits (i.e., Advanced timing can substantially alter sensor values.)
Step | Action | Value(s) | Yes | No | ||||||||
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Important: Check for applicable service bulletins before proceeding with this test. Perform this test only if there is a driveability complaint or if you have been sent to this table from another section in the service information. Important: Do not turn the ignition OFF when performing this diagnostic. Do not clear the DTCs unless instructed by this diagnostic. Turn on the ignition leaving the engine OFF Does the scan tool power up? | -- | |||||||||||
Turn ON the ignition leaving the engine OFF. Is the MIL ON? | -- | |||||||||||
3 | Attempt to establish communications with the VCM. Does the scan tool communicate with the VCM? | -- | ||||||||||
Save the DTC Freeze Frame and the Failure Records if any of the following DTC status are set in the scan tool DTC Information:
Did you find any DTCs? | -- | Go to the applicable DTC table | ||||||||||
Monitor the Serial Data Class II messages with the scan tool. Are all other modules communicating with the scan tool? | -- | |||||||||||
6 | Attempt to start the engine. Does the engine start and continue to run? | -- | ||||||||||
Command the MIL OFF with the scan tool. Does the MIL turn OFF? | -- | |||||||||||
Compare the scan tool engine data with the TYPICAL scan tool values. Are the values within the typical ranges? | -- | Go to Symptoms | Go to Diagnostic Aids |
