The alternative fuels engine control unit (AF ECU) controls alternative fuel engine operation. The control unit monitors various engine and vehicle functions to provide the correct amount of fuel under all operating conditions. This provides excellent driveability and fuel economy while maintaining exhaust emission levels. Refer to Engine Controls Component Views for location views.

In addition to controlling engine operation the AF ECU also runs tests (DTCs) on various sensors and outputs. If a DTC detects a problem the AF ECU will store the DTC number along with sensor information retrieved at the time of the problem (Freeze Frame). The DTC number and any Freeze Frame information can be retrieved from the AF ECU using a scan tool.

The AF ECU operates in several Function modes which are defined in the AF ECU Function Parameters table.

DTCs, when set, indicate that a control module (AF ECU, PCM/VCM or ECM) has detected a malfunction in a particular circuit or system. The control module is programmed with routines or checks that it follows only under prescribed conditions (called Conditions for Running the DTC). When these conditions exist, the control module checks certain circuits or systems for a malfunction. These checks are called Conditions for Setting the DTC. When the setting conditions are true, a malfunction is indicated and the DTC is stored as failed. Some DTCs alert the driver by illuminating the SERVICE ENGINE SOON MIL (some vehicles may also display a message). Other DTCs don't trigger a driver warning. Refer to the Diagnostic Trouble Code (DTC) List table in Specifications for a complete list of AF ECU DTCs and what driver alerts they trigger. The control modules also saves data and input parameters when most DTCs are set. The saved information is called Freeze Frame and Failure Records. The AF ECU does not store Failure Records.

The ability for a DTC to run depends largely upon whether or not a Trip has been completed. A Trip for a particular DTC is defined as vehicle operation, followed by an engine off period and a driving mode such that any particular DTC has had sufficient time to complete testing. The requirements for trips vary as they may involve items of an unrelated nature; driving style, length of trip, ambient temperature, etc. Some DTCs run only once per trip (e.g. catalyst monitor) while others run continuously (e.g. misfire and fuel system monitors). If the proper enabling conditions are not met during that ignition cycle, the tests may not be complete or the test may not have run.

In addition, the execution of a DTC may also be bound by conditions which must comprehend a Warm-up cycle. A Warm-up cycle consists of engine start-up and vehicle operation such that the coolant temperature has risen greater than a certain value typically 40°C (104°F) from start-up temperature and reached a minimum temperature of 70°C (160°F). If this condition is not met during the ignition cycle, the DTC may not run.

Government regulations require that engine operating conditions be captured whenever the MIL is illuminated. The data captured is called Freeze Frame data. The Freeze Frame data is very similar to a single record of operating conditions. Whenever the MIL is illuminated, the corresponding record of operating conditions is recorded as Freeze Frame data. A subsequent failure will not update the recorded operating conditions.

The Freeze Frame data parameters stored with a DTC failure include the following:

Freeze frame data can only be overwritten with data associated with a misfire or fuel trim malfunction. Data from these faults take precedence over data associated with any other fault. The Freeze Frame data will not be erased unless the associated history DTC is cleared.

In addition to Freeze Frame data some control modules may also store Failure Records data when a DTC reports a failure. Unlike Freeze Frame data, Failure Records data can be stored by DTCs that DO NOT illuminate the MIL. Only the gasoline Control Module (VCM/PCM or ECM) stores Failure records. The AF ECU does NOT store Failure Records.

Failure Records save many more relevant data parameters than Freeze Frame records. Which data parameters are saved varies based on the DTC set. Sensor DTC Failure Records generally contain sensor related data parameters while fuel DTC Failure Records generally contain fuel related data parameters. If several DTCs are set, Failure Records for the most recently set DTCs will be saved.

Freeze Frame, Failure Records data may be retrieved through the Diagnostic Trouble Code menu on scan tool. If more than one DTC is set review the odometer or engine run time data located in the Freeze Frame, Failure Records info to determine the most current failure.

Keep in mind that once Freeze Frame or Failure Record is selected, the parameter and input data displayed will look just like the normal data except the parameters will not vary since it is displaying recorded data.

Important: Always capture the Freeze Frame and Failure Records information with the scan tool BEFORE proceeding with diagnosis. Clearing DTCs, disconnecting the battery, disconnecting control module or body connectors or procedures performed during diagnosis may ERASE or overwrite the stored Freeze Frame and Failure Records data. Loss of this data may prevent accurate diagnosis of an intermittent or difficult to set DTC.

Selecting this option on the scan tool allows the technician to record the Freeze Frame/Failure Records data that may be stored in the control module memory. This can be useful if the control module connectors or the vehicle battery must be disconnected and later review of the stored information may be desired.

DTCs are categorized by type. The type will indicate the action the DTC will take in storing a failure and illuminating the MIL. The following table indicates what action each DTC type will take when a failure is recorded. Type C DTCs that DO NOT display a message were formerly referred to as Type D.

In order for a type B DTC to request MIL illumination the DTC must fail in two consecutive drive trips in which the DTC tests.

Refer to the Diagnostic Trouble Code (DTC) List table in Specifications for the type of each AF ECU DTC.

Always refer to the diagnostic support information within each DTC to obtain the Action Taken when the DTC sets and the Conditions for Clearing the DTC. These will indicate any variations from the general type A, B and C actions.

Fuel trim and misfire are special cases of type B DTCs. Each time a fuel trim or misfire malfunction is detected, engine load, engine speed, and engine coolant temperatures are recorded. In order for the fuel trim or misfire DTCs to report a PASS the load conditions must be within 10 percent, the speed conditions must be within 375 rpm, and the coolant temperatures must be in the same calibrated high or low range at the time the diagnostic test last reported a failure.

When the ignition is turned OFF, the last reported set of conditions remain stored. During subsequent ignition cycles, the stored conditions are used as a reference for similar conditions. If a malfunction occurs during two consecutive trips, the control module treats the failure as a normal type B diagnostic, and does not use the stored conditions. However, if a malfunction occurs on two non-consecutive trips, the stored conditions are compared with the current conditions. The MIL will then illuminate under the following conditions:

Unique to the misfire diagnostic, the misfire DTC has the ability of alerting the vehicle operator to potentially damaging levels of misfire. If a misfire condition exists that could potentially damage the catalytic converter as a result of high misfire levels, the control module will command the MIL to flash at a rate of once per second during the time that the catalyst damaging misfire condition is present.

Snapshot data may also be taken and retrieved using a diagnostic tool by selecting the Snapshot option. These parameters can then be reviewed at any time.

Keep in mind that once a Snapshot is triggered, the parameter and input data displayed will look just like the normal control module parameter and input data with the same parameter numbers except it will not vary since it is displaying recorded data.

DTCs can only be displayed with the use of a scan tool.

This selection will display any DTCs that have not run during the current ignition cycle or have reported a test failure during this ignition cycle. DTC tests which run and pass will cause that DTC number to be removed from this scan tool display.

The scan tool will display any DTCs which have failed at any time since DTCs were last cleared.

The display will indicate a DTCs status for the LAST TEST, during THIS IGNITION and SINCE CLEAR.

ALWAYS use a scan tool to clear DTCs from control module memory. Disconnecting the vehicle battery MAY also clear the control module memory, however most other system memories will also be cleared. This is generally undesirable. Disconnecting the control module connectors solely for clearing DTCs is not recommended since this unnecessarily disturbs the connections. Before clearing DTCs the scan tool has the capability to save any data stored with the DTCs and then display that data at a later time. Refer to Capture Info. Once a problem has been corrected and verified it is a good idea to clear DTCs so that any future service work is not needlessly confused.

Many DTCs have complex running and setting conditions. Therefore, simply clearing DTCs and watching to see if the DTC sets again may not indicate whether a problem has been corrected. To verify a repair after it is complete, you must look up the test conditions and duplicate those conditions. If the DTC runs and passes, chances are good that the problem is fixed.

The provision for communicating with a control module is the data link connector (DLC). It is usually located under the instrument panel. The DLC is used to connect to a scan tool. Some common uses of the scan tool are listed below:

U.S. Federal regulations require that all automobile manufacturers establish a common communications system. General Motors utilizes the Class II communications system. Each bit of information can have one of two lengths: long or short. This allows vehicle wiring to be reduced by the transmission and reception of multiple signals over a single wire. The messages carried on Class II data streams are also prioritized. In other words, if two messages attempt to establish communications on the data line at the same time, only the message with higher priority will continue. The device with the lower priority message must wait. The most significant result of this regulation is that it provides scan tool manufacturers with the capability of accessing data from any make or model vehicle sold in the United States.

The SERVICE ENGINE SOON Malfunction Indicator Lamp (or MIL) is located in the instrument panel cluster (IPC). The MIL is controlled both by the AF ECU and the gasoline control module (VCM/PCM or ECM). The MIL is used to indicate that a control module (AF ECU, VCM/PCM or ECM) has detected a problem that affects vehicle emissions, may cause powertrain damage, or severely impacts driveability.

Refer to the Engine Controls Description and Operation section in the C/K service manual for further information that may apply to this vehicle.