GM Service Manual Online
For 1990-2009 cars only

Description

Due to the nature of the vehicles designed to run on fuels other than gasoline, some additional procedures may be required in order to set the I/M System Status indicators to YES. Vehicles designed to run on both gasoline and compressed natural gas (CNG) will disable certain diagnostic tests when the vehicle is operating on CNG. These vehicles will require operation using gasoline in order for the remaining tests to run. The System Set procedures for gasoline vehicles can be used once the vehicle is operating on gasoline.

Conditions for Updating the I/M System Status

Each system requires at least one, and sometimes several, diagnostic tests. The results of these tests are reported by a diagnostic trouble code (DTC). A system monitor is complete when either all of the DTCs comprising the monitor have Run and Passed, or any one of the DTCs comprising the monitor have illuminated the MIL. Once all of the tests are completed, the I/M System Status display will indicate YES in the Completed column. For example, when the HO2S Heater Test indicates YES, all of the oxygen sensor heaters have been diagnosed. If the vehicle has four heated oxygen sensors, all four heater circuits have been diagnosed. The I/M System Status will indicate NO under the Completed column when any of the required tests for that system have not run. The following is a list of conditions that would set the I/M System Status indicator to NO:

    • The vehicle is new from the factory and has not yet been driven through the necessary drive conditions to complete the tests.
    • The battery has been disconnected or discharged below operating voltage.
    • The control module power or ground has been interrupted.
    • The control module has been reprogrammed.
    • The control module DTCs have been cleared as part of a service procedure.

Monitored Emission Control Systems

The CNG controller supports HO2S monitoring. The I/M tests that are supported by the gasoline controller are the same as those in a non bi-fuel vehicle.

Diagnostic Aids

The I/M System Status display provides an indication of when the control module has completed the required DTC tests. This does not necessarily mean that the test has passed, only that a decision was made. If the diagnostic fails, a DTC will indicate the failure. If a failure indication is present for a DTC associated with one of the I/M regulated systems, it may prevent other required tests from running. For example, a DTC for the control circuit of the relay controlling an AIR pump may not be listed in the Inspection/Maintenance System DTC Table, because it is a continuous test. If this DTC is set, the Active Tests for the AIR system may not run.

The I/M System Status information may be useful for a technician to determine if diagnostics have run when verifying repairs.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. Any DTCs set, even those that are not listed in the Inspection/Maintenance System DTC Table, may prevent the required DTCs from running. If there is any question as to whether a set DTC is disabling the required I/M diagnostic, review the Conditions for Running in the diagnostic procedures for the DTC required by the I/M diagnostic. A list of disabling DTCs, if applicable, is contained in the supporting text for that DTC.

  2. Anytime a control module is reprogrammed or the diagnostic trouble codes are cleared as part of a repair procedure, all the I/M System Status indicators will reset to NO.

  3. This step is to determine if the system status that was not complete is in the gasoline control module or the CNG control module.

  4. Removing the fuse for the CNG control module will force the vehicle to run on gasoline. Once the vehicle is running on gasoline, the set procedures for gasoline fueled vehicles can be used to set the I/M System Status to YES. Use discretion when determining whether the entire I/M Complete System Set Procedure needs to be performed. For example, if the only tests that have not run are those that require the engine to be at operating temperature, then only those individual tests need to be run. There is no need to allow the engine to completely cool in order to run these tests. Removing the CNG fuse will reset the I/M System Status in the CNG controller to NO.

  5. Reinstalling the fuse for the CNG control module will restore CNG operation.

  6. This step is to determine if a system status is not complete in the CNG control module.

Step

Action

Yes

No

1

  1. Perform Diagnostic System Check - Alternative Fuels (Engine Controls) .
  2. Important: Many DTC related repairs will instruct the technician to clear the DTC information. This procedure will reset ALL of the I/M System Status indicators to NO, and require performing the I/M Complete System Set Procedure.

  3. Repair any DTCs or driveability concerns that would prevent the I/M System Status tests from completing.

Did you find and repair a DTC or driveability concern?

Go to Step 3

Go to Step 2

2

  1. Review any service bulletins for software updates that may prevent I/M readiness.
  2. Perform any reprogramming or repairs indicated by the service bulletins.

Is the action complete?

Go to Step 3

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3

Using the scan tool, observe the I/M System Status display for the gasoline control module.

Are any of the tests indicating NO?

Go to Step 4

Go to Step 6

4

  1. Turn OFF the ignition.
  2. Remove the fuse for the CNG control module.
  3. Start the engine and allow it to idle.
  4. Using the scan tool, verify that the engine is operating in gasoline mode.
  5. Perform the necessary set procedures for the I/M System Status indicators that were set to NO.

Is the action complete?

Go to Step 5

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5

  1. Turn OFF the ignition.
  2. Reinstall the fuse for the CNG control module.
  3. Start the engine and allow it to idle.
  4. Using the scan tool, verify that the engine is operating in CNG mode.
  5. Perform the necessary set procedures for the I/M System Status indicators that were set to NO.

Is the action complete?

System OK

--

6

  1. Start the engine and allow it to idle.
  2. Using the scan tool, verify that the engine is operating in CNG mode.
  3. Perform the necessary set procedures for the I/M System Status indicators that were set to NO.

Is the action complete?

System OK

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