Common OBD II Terms
Common OBD II TermsDiagnostic
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. There are many diagnostics, shown in the following list:
^ Misfire
^ Heated Oxygen Sensor 1 (HO2S1)
^ Heated Oxygen Sensor 2 (HO2S2)
^ Exhaust Gas Recirculation (EGR)
^ Catalyst monitoring
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."
The enable criteria for each diagnostic is listed on the first page of the Diagnostic Trouble Code (DTC) description under the heading "Conditions for Setting the DTC." Enable criteria varies with each diagnostic and typically includes, but is not limited to, the following items:
^ Engine speed
^ Vehicle speed
^ Engine Coolant Temperature (ECT)
^ Manifold Absolute Pressure (MAP)
^ Barometric Pressure (BARO)
^ Intake Air Temperature (IAT)
^ Throttle Position (TP)
^ High canister purge
^ Fuel trim
^ A/C ON
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 diagnostics are run when the vehicle is at operating temperature, some when the vehicle first starts up; some require that the vehicle be cruising at a steady highway speed, some run only when the vehicle is at idle; some diagnostics function with the Torque Converter Clutch (TCC) disabled. Some run only immediately following a cold engine startup.
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.
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) is illuminated.
Malfunction Indicator Lamp (MIL)
The Malfunction Indicator Lamp (MIL) is required by On-Board Diagnostics (OBD II) that it illuminates under a strict set of guide lines.
Basically, the MIL is turned ON when the Powertrain Control Module (PCM)/Engine Control Module (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.
Extinguishing the MIL
When the MIL is ON, the Diagnostic Executive will turn OFF the MIL after three consecutive trips that a "test passed" has been reported for the diagnostic test that originally caused the MIL to illuminate. Although the MIL has been turned OFF, the DTC will remain in the PCM/ECM memory (both Freeze Frame and Failure Records) until forty (40) consecutive warm up cycles after no faults have been completed.
If the MIL was set by either a fuel trim or misfire-related DTC, additional requirements must be met. In addition to the requirements stated in the previous paragraph, these requirements are as follows:
^ The diagnostic tests that are passed must occur with 375 rpm of the RPM data stored at the time the last test failed.
^ Plus or minus 10% of the engine load that was stored at the time the last test failed. Similar engine temperature conditions (warmed up or warming up) as those stored at the time the last test failed.
Meeting these requirements ensures that the fault which turned ON the MIL has been corrected.
The MIL 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 system check, the MIL 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, an OBD II System Check must be performed. The procedures for these checks are given in OBD II System Check. These checks will expose faults which may not be detected if other diagnostics are performed first.
Data Link Connector (DLC)
The provision for communicating with the control module is the Data Link Connector (DLC). The DLC is used to connect to a scan tool. Some common uses of the scan tool are listed below:
^ Identifying stored DTCs
^ Clearing DTCs
^ Performing output control tests
^ Reading serial data