Diagnostic Information

Special Cases Of Type A Diagnostic Tests
Unique to the misfire diagnostic, the diagnostic executive has the capability of alerting the driver of potentially damaging levels of misfire. If a misfire condition exists that could potentially damage the catalytic converter, the diagnostic executive will command the MIL to flash at a rate of once per second during the times that the catalyst damaging misfire condition is present.

Special Cases Of Type B Diagnostic Tests
Misfire and fuel trim malfunctions are special cases of type B diagnostics. Each time a fuel trim malfunction is detected, the engine load, the engine speed, and the engine coolant temperatures are recorded.

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 fuel trim malfunction occurs during 2 consecutive trips, the Diagnostic Executive treats the failure as a normal type B diagnostic. The Diagnostic Executive does not use the stored conditions. However, if a fuel trim malfunction occurs on 2 non-consecutive trips, the stored conditions are compared with the current conditions. The MIL will then illuminate under the following conditions:
- When the engine load conditions are within 10 percent of the previous test that failed.
- The engine speed is within 375 RPM of the previous test that failed.
- The engine coolant temperature is in the same range as the previous test that failed.

Storing And Erasing Freeze Frame Data
Government regulations require that the engine operating conditions are to be captured whenever the MIL is illuminated. The data that is 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 to the Freeze Frame buffer.

Each time a diagnostic test reports a failure, the current engine operating conditions are recorded in the failure records buffer. A subsequent failure will update the recorded operating conditions. The following operating conditions for the diagnostic test which failed typically include the following parameters:
- The air fuel ratio
- The air flow rate
- The fuel trim
- The engine speed
- The engine load
- The engine coolant temperature
- The vehicle speed
- The TP angle
- The MAP/BARO
- The injector base pulse width
- The loop status

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

Intermittent Malfunction Indicator Lamp
In the case of an intermittent fault, the malfunction indicator lamp (MIL) may illuminate and then after 3 trips turn OFF. However, the corresponding diagnostic trouble code will store in the memory. When unexpected diagnostic trouble codes appear, check for an intermittent malfunction.

Data Link Connector (DLC)
The provision for communicating with the control module is a Data Link Connector (DLC). The DLC is usually located under the instrument panel. The DLC is used in order to connect to a scan tool. Some common uses of the scan tool are listed below:
- Identifying stored Diagnostic Trouble Codes (DTCs).
- Clearing the DTCs
- Performing the output control tests.
- Reading the serial data.

Control Module Learning Ability
The control module has a learning ability which allows the control module to make corrections for minor variations in the fuel system in order to improve driveability. Whenever the battery cable is disconnected, the learning process resets.

The driver may note a change in vehicle performance. In order to allow the VCM to re-learn, drive the vehicle at part throttle with moderate acceleration.

Reprogramming (Flashing) The Control Module
Some vehicles allow the reprogramming of the control module without removal from the vehicle. This provides a flexible and a cost-effective method of making changes in software and calibrations. Refer to the latest Techline information on reprogramming or flashing procedures.