On-Board Diagnostic (OBD)
ON-BOARD DIAGNOSTIC (OBD)On-Board Diagnostic Tests
A diagnostic test is a series of steps, the result of which is a pass or fail reported to the diagnostic executive. When a diagnostic test reports a pass result, the diagnostic executive records the following data:
- The diagnostic test has been 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 been completed since the last ignition cycle.
- 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.
Passive and Active Diagnostic Tests
A passive test is a diagnostic test which simply monitors a vehicle system or component. Conversely, an active test, actually takes some sort of action when performing diagnostic functions, often in response to a failed passive test.
Intrusive Diagnostic Tests
This is any on-board test run by the Diagnostic Management System which may have an effect on vehicle performance or emission levels.
Warm-Up Cycle
A warm-up cycle means that engine at temperature must reach a minimum of 70 °C (1600 °F) and rise at least 22 °C (40 °F) over the course of a trip.
Common OBD Terms
Diagnostic
When used as a noun, the word diagnostic refers to any on-board test run by the vehicle's Diagnostic Management System. A diagnostic is simply a test run on a system or component to determine if the system or component is operating according to specification.
Enable Criteria
The term "enable criteria" is engineering language for the conditions necessary for a given diagnostic test to run. Each diagnostic has a specific list of conditions which must be met before the diagnostic will run. "Enable criteria" is another way of saying "conditions required".
Trip
Technically, a trip is a key on-run-key off cycle in which all the enable criteria for a given diagnostic are met, allowing the diagnostic to run. Unfortunately, this concept is not quite that simple. A trip is official when all the enable criteria for a given diagnostic are met But because the enable criteria vary from one diagnostic to another, the definition of trip varies as well. Some diagnostic are run when the vehicle is at operating temperature, some when the vehicle first start up; some require that the vehicle be cruising at a steady highway speed, some run only when the vehicle is idle. Some run only immediately following a cold engine start-up.
A trip then, is defined as a key on-run-key off cycle in which the vehicle was operated in such a way as to satisfy the enables criteria for a given diagnostic, and this diagnostic will consider this cycle to be one trip. However, another diagnostic with a different set of enable criteria (which were not met) during this driving event, would not consider it a trip. No trip will occur for that particular diagnostic until the vehicle is driven in such a way as to meet all the enable criteria
The Diagnostic Executive
The Diagnostic Executive is a unique segment of software which is designed to coordinate and prioritize the diagnostic procedures as well as define the protocol for recording and displaying their results. The main responsibilities of the Diagnostic Executive are listed as following:
- Commanding the MIL ("Check Engine" lamp) on and off
- DTC logging and clearing
- Non-emission related Service Lamp (future)
- Current status information on each diagnostic
- System Status
The Diagnostic Executive records DTCs and turns on the MIL when emission-related faults occur. It can also turn off the MIL if the conditions cease which caused the DTC to set.
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. 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) is illuminated.
Malfunction Indicator Lamp (MIL)
The Malfunction Indicator Lamp (MIL) looks the same as the MIL you are already familiar with ("Check Engine" lamp). However, OBD requires that the it illuminate under a strict set of guide lines. Basically, the MIL is turned on when the ECM detects a DTC that will impact the vehicle emissions.
The MIL is under the control of the Diagnostic Executive. The MIL will be turned on if an emissions-related diagnostic test indicates a malfunction has occurred. It will stay on until the system or component passes the same test, for three consecutive trips with no emissions related faults.
Data Link Connector (DLC)
The provision for communication with the control module is the Data Link Connector (DLC). It is located at the lower left of the instrument panel behind a small square cover. The DLC is used to connect to a scan tool. Some common uses of the scan tool are listed below:
- Identifying stored Diagnostic Trouble Codes (DTCs).
- Clearing DTCs.
- Performing output control tests.
- Reading serial data.
ON-BOARD DIAGNOSTIC (OBD) System Check
OBD System should be checked as follows:
1. When Ignition key is turned from the "OFF" to the "ON" position, make sure that MIL is lit for 0.3 sec. to 0.8 sec.
2. Connect Scan Tool and check to see if MIL is always lit.
If so, OBD System is normal.