- Description of On-Board Diagnostics

On Board Diagnostic Tests
A diagnostic test is a series of steps which has a beginning and end. The result of which is a pass or fail reported to the Diagnostic Executive (Refer to Serial Data Communications under this heading for additional information.). When a diagnostic test reports a pass result, the Diagnostic Executive records the following data:

^ The diagnostic test has completed since the last ignition cycle.
^ The diagnostic test has passed during the current ignition cycle.
^ The fault identified by the diagnostic test is not currently active.

When a diagnostic test reports a fail result, the Diagnostic Executive records the following data:

^ The diagnostic test has completed since the last ignition.
^ The fault identified by the diagnostic test is currently active.
^ The fault has been active during this ignition cycle.
^ The operating conditions at the time of the failure.

Diagnostic Trip
The ability for a diagnostic test to run depends largely upon whether or not a "Trip" has been completed. A "Trip" for a particular diagnostic is defined as vehicle operation, followed by an engine off period of duration and driving mode such that any particular diagnostic test has had sufficient time to complete at least once. The requirements for trips vary as they may involve items of an unrelated nature; driving style, length of trip, ambient temperature, etc. Some diagnostic tests 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.

Warm-up Cycle
In addition, the execution of diagnostic tests 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 40°F from start-up temperature and reached a minimum temperature of 160°F. If this condition is not met during the ignition cycle, the diagnostic may not run.

Diagnostic Information
The diagnostic charts and functional checks are designed to locate a faulty circuit or component through a process of logical decisions. The charts are prepared with the requirement that the vehicle functioned correctly at the time of assembly and that there are not multiple faults present.

There is a continuous self-diagnosis on certain control functions. This diagnostic capability is complemented by the diagnostic procedures contained in this manual. The language of communicating the source of the malfunction is a system of diagnostic trouble codes. When a malfunction is detected by the control module, a diagnostic trouble code is set and the Malfunction Indicator Lamp (MIL) "Service Engine Soon" is illuminated on some applications.

Malfunction Indicator Lamp
The MIL (Service Engine Soon) is on the instrument panel and has the following functions:

^ It informs the driver that a fault that affects vehicle emission levels has occurred and that the vehicle should be taken for service as soon as possible.

^ As a bulb and system check, the MIL (Service Engine Soon) will come "ON" with the key "ON" and the engine not running. When the engine is started. the MIL will turn "OFF."

When the MIL remains "ON" while the engine is running, or when a malfunction is suspected due to a driveability or emissions problem, a Powertrain On-Board Diagnostic (OBD) System Check must be performed. These checks will expose faults which may not be detected if other diagnostics are performed first.

MIL Requests and History Codes
(What happens when type A, B, C and D DTCs report failures and passes, and how the MIL responds. Includes criteria for turning the MIL "ON" and "OFF".)

The Diagnostic Executive must be able to acknowledge when all emissions related diagnostic tests have reported a pass or fail condition since the last ignition cycle. Diagnostic tests are separated into different types:

^ A - emissions related, turns "ON" the MIL,

^ B - emissions related, turns "ON" the MIL if a fault is active for 2 consecutive driving cycles (except for misfire and fuel trim DTCs),

^ C - non-emissions related does not turn "ON" the MIL, but may turn "ON" a "Service Lamp" if the vehicle is so equipped.

^ D - non-emissions related, does not turn "ON" any lamp. D non-emissions related may not be utilized on certain vehicle applications.

When a type A diagnostic test reports a failure, the Diagnostic Executive initiates a request to have the MIL turn on for that diagnostic test. When a type B diagnostic test reports a second failure, the Diagnostic Executive initiates a request for that diagnostic test. A type D failure records the DTC but does not illuminate the MIL. The Diagnostic Executive has the option of turning the MIL "OFF" when three consecutive trips are recorded where the diagnostic system passes the test. In the case of misfire or fuel trim malfunctions, there is an additional requirement that the load conditions must be within 10%, the speed conditions must be within 375 rpm, and the coolant temperatures must be in the same calibration high or low range at the time the diagnostic test last reported a failure. For a type C diagnostic test, the Diagnostic Executive will request that the Auxiliary Service Lamp (if so equipped) be turned off at the next ignition key-on cycle or test passing report.

When the Diagnostic Executive requests the MIL to be turned "ON," a history DTC is also recorded for the diagnostic test. To clear a history DTC for any diagnostic tests requires 40 subsequent warm-up cycles during which no diagnostic tests have reported a fail.

Special Cases Of Type B Diagnostic Tests
Unique to the misfire diagnostic, the Diagnostic Executive has the capability 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 Diagnostic Executive will command the MIL to "flash" at a rate of once per second during those the time that the catalyst damaging misfire condition is present.

Fuel trim and misfire are special cases of type B diagnostics. Each time a fuel trim or misfire malfunction is detected, engine load, engine speed, and engine coolant temperatures are recorded.