Circuit Description
The on-board diagnostic (OBD) system check is an organized approach to identifying a problem created by an electronic engine control system malfunction.
The OBD system check MUST be the starting point for any driveability concern diagnosis. The OBD system check will direct you to the next logical step in diagnosis.
The OBD system check will identify failures in both the alternative fuel and the gasoline engine control systems.
Important: Only perform this check if a driveability concern is present or if you were sent here from other service information.
Diagnostic Aids
Important: System voltage must remain between 9-16 volts for serial data communication.
Always store control module (AF ECU, VCM / PCM or ECM) DTC information into a scan tool before proceeding with diagnosis. The stored control module DTC information may contain data (Freeze Frame or Failure Records) useful for intermittent diagnosis. Clearing DTC information with a scan tool or the loss of control module power or ground will erase the control module DTC information. Some diagnostic procedures call for disconnection of control module connectors and loss of control module stored DTC information may occur.
Test Description
The number(s) below refer to the step number(s) on the diagnostic table.
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This step checks for proper initial MIL operation.
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If the scan tool fails to turn ON, check scan tool operation on a known good vehicle. The data link connector (DLC) supplies the scan tool with a power circuit (terminal 16) and two ground circuits (terminals 4 and 5). All three circuits are required for proper scan tool operation.
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Both control modules simultaneously test certain DTCs regardless of the fuel system that is in operation.
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Checking for a shorted MIL circuit and proper control module operation of the MIL circuit.
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The fuel indicator lamp (FIL) located within the fuel gauge select switch button will illuminate when the engine is operating on gasoline.
In addition the NGO Enable Output scan tool parameter will indicate the AF ECU's commanded state of the NGO Enable circuit while the NGO Enable Input parameter will indicate the gasoline control modules monitored state of the NGO Enable circuit. Regardless of the fuel system in operation these two data parameters will always indicate the same data unless an NGO Enable circuit fault is present.
If the vehicle is operating on GASOLINE and the FIL is NOT illuminated refer to Fuel Gauge Selector Switch/Fuel Indicator Lamp Diagnosis .
If the NGO Enable Input and Output parameters do not indicate the same data refer to Fuel System Unwanted Switch to Gasoline .
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If actual and typical data values are different and a fault is suspected check the DTC information with a scan tool and verify that the suspected component or system DTCs have ran and passed. If they have passed use the sensor information in Description and Operation to help diagnose suspected sensor faults. If a system fault is suspected (EGR operation, EVAP operation, etc.) refer to the DTC that applies to the system and if possible use the diagnostic table to aid in diagnosis.
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Check for applicable bulletins before proceeding with DTC diagnosis.
After determining which control module has stored DTCs, diagnose DTCs in the following order:
| • | PCM MEMORY or INTERNAL FAULT DTCs (PCM Memory, PCM Not Programmed, etc.) |
| • | SYSTEM and IGNITION VOLTAGE DTCs (System Voltage High or Low, etc.) |
| • | COMPONENT / CIRCUIT DTCs (TP Sensor Circuit Voltage High or Low, Oxygen Sensor Circuit Insufficient Activity, etc.) |
| • | SYSTEM DTCs (MAP System Performance, TP Sensor Performance, etc.) |
Step | Action | Value(s) | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
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Important: Inspect for applicable service bulletins before proceeding with this test. Ensure BOTH fuel tanks are at least ¼ full.Do not clear DTCs unless instructed by a diagnostic procedure.
Is the MIL on? | -- | Go to Step 2 | ||||||||
Using a scan tool attempt to communicate and retrieve data from the gasoline control module (VCM / PCM or ECM). If the scan tool fails to turn ON, CHECK SCAN TOOL OPERATION ON A KNOWN GOOD VEHICLE. Does the scan tool display gasoline control module data? | -- | Go to Step 3 | Go to Powertrain On Board Diagnostic (OBD) System Check in the gasoline Service Manual | |||||||
3 | Using a scan tool attempt to communicate and retrieve data from the alternative fuels engine control unit (AF ECU). Does the scan tool display AF ECU data? | -- | Go to Step 4 | Go to Ignition Relay Diagnosis | ||||||
Were any DTCs stored? | -- | Go to Step 10 | ||||||||
Command the MIL OFF utilizing the scan tool Engine Output (Gasoline) for the MIL. Does the MIL turn OFF? | -- | |||||||||
6 | Crank the engine for at least ten seconds. Does the engine start? | -- | ||||||||
Determine which fuel system the engine is operating on. . Is the engine operating on Alternative Fuel? | -- | Go to Step 8 | ||||||||
Are the values normal or within typical range? | -- | Go to applicable DTC diagnosis | ||||||||
9 |
Does the engine start and continue to run with the CNG 20 amp fuse removed? | -- | Go to Ignition Relay Diagnosis | |||||||
Have all DTCs been diagnosed? | -- | Go to applicable DTC diagnosis. | ||||||||
11 | Is there a driveability symptom? | -- | ||||||||
12 | Perform the fuel system diagnosis and return to this step. Refer to Fuel System Diagnosis . Did the fuel system diagnosis locate and repair a concern? | -- | Go to Symptoms | |||||||
13 | Is there a fuel gauge, fuel gauge selector switch or fuel indicator lamp operation concern? | -- | Go to Fuel Gauge Selector Switch/Fuel Indicator Lamp Diagnosis | |||||||
14 |
Does the engine start and run on gasoline? | -- | ||||||||
15 |
Does the vehicle operate on CNG with normal driveability, no FIL/MIL illumination and no stored DTCs? | -- | System OK |
