GLOSSARY OF TERMS

The following terms are related to on-board diagnostic systems. Read and reference this list as needed to aid in the understanding of OBD2 systems.

CARB 每 California Air Resources Board

※Check Engine§ Light- See ※Malfunction Indicator Lamp (MIL)§

Communication Protocol 每 The method or means by which data is transferred between the vehicle*s on-board computer and an external data retrieval device, such as a Code Reader or Scan Tool.

Computer Control System 每 An electronic control system, consisting of an on-board computer and related sensors, actuators and switches, used to ensure peak performance and fuel efficiency while reducing pollutants in the vehicle*s emissions. OBD1 and OBD2 systems are computer control systems.

Continuous Monitor 每 See ※:Monitors§

Data Link Connector (DLC) 每 An electrical connector that provides the means of accessing and retrieving data from the vehicle*s on-board computer. OBD1 systems employ a variety of manufacturer-specific connectors. OBD2 Systems use a standardized 16-pin DLC, as defined by SAE J1962/ISO 15031-3.

Diagnostic Trouble Code (DTC) - Numeric (OBD1 systems) or alpha-numeric (OBD2 systems) codes that are used to identify a problem that is present in any of the systems that are monitored by the vehicle*s on-board computer. In OBD1 systems, DTCs are manufacturer- and/or model-specific. In OBD2 systems, DTCs can be ※generic§ or ※manufacturer-specific.§

-          Generic DTC 每 A DTC that applies to all OBD2-compliant vehicles.

-          Manufacturer-Specific DTC 每 A DTC that applies only to OBD2-compliant vehicle*s made by a specific manufacturer.

Driving Condition 每 A specific environmental or operational condition under which a vehicle is driven (operated); such as starting the vehicle when cold, driving at steady speed (cruising), accelerating, etc.


Enabling Criteria - Each Monitor is designed to test and monitor the operation of a specific part of the vehicle's emissions system (EGR system, oxygen sensor, catalytic converter, etc.). A specific set of "conditions" or "driving procedures" must be met before the computer can command a Monitor to run tests on its related system. These "conditions" are known as "Enabling Criteria." The requirements and procedures vary for each Monitor. Some Monitors only require the ignition key to be turned "On" for them to run and complete their diagnostic testing. Others may require a set of complex procedures, such as, starting the vehicle when cold, bringing it to operating temperature, and driving the vehicle under specific conditions before the Monitor can run and complete its diagnostic testing.

EPA 每 Environmental Protection Agency

Freeze Frame Data 每 A digital representation or ※snapshot§ of engine and/or emissions system conditions present when a given Diagnostic Trouble Code (DTC) was stored. The vehicle*s computer stores Freeze Frame Data for the priority DTC only.

Fuel Trim 每 A vehicle requires a mixture of air and fuel to achieve combustion. The optimal mixture of air and fuel is known as the stoichiometric ratio, and is generally defined as 14.7:1 times the mass of air to fuel. To ensure optimum engine performance with the least amount of pollution, the vehicle*s computer constantly monitors the air/fuel mixture, and adjusts it according to current driving requirements. The vehicle*s computer stores predetermined ※reference§ values for air/fuel ratio for all possible driving conditions (base fuel trim). The computer receives inputs from sensors (primarily the Engine Coolant Temperature Sensor and Oxygen Sensor) and compares the ※actual§ values from these sensors with the programmed reference values. If the sensor input values do not match the ※reference§ values for that particular driving condition, the computer command the proper components (actuators) to make corrections (fuel trim adjustments). Fuel trim adjustments can be ※short term§ or ※long term.§

-          Short Term Fuel Trim (STFT) 每 The vehicle*s computer makes adjustments to the base fuel trim program by adding or subtracting fuel from the base program (to achieve the optimum air/fuel ratio) based on inputs from the vehicle*s Oxygen Sensor(s).

-          Long Term Fuel Trim (LTFT) - The vehicle*s computer makes adjustments to the base fuel trim program by adding or subtracting fuel from the base program (to achieve the optimum air/fuel ratio) based on the average of STFT fuel corrections. LTFT adjustments often mask underlying problems such as MAF sensor problems, leaking fuel injectors, leaking intake manifold gaskets, etc.


Generic DTC 每 See ※Diagnostic Trouble Code (DTC)§

I/M Readiness 每 I/M is an Inspection and Maintenance program legislated by the Government to me federal clean-air standards. The program requires that a vehicle be taken to an ※Emissions Test Station§ for an ※Emissions Test§ or ※Smog Check,§ where the emissions-related components and systems are inspected and tested for proper operation.

I/M Readiness Status 每 I/M Readiness status is determined by checking the state of a vehicle*s Monitors for ※has run§ or ※has not run§ status. If all supported Monitors for a vehicle show a ※has run§ status it indicates that all monitored emissions-related components and systems are operating properly; the vehicle is ready for an Emissions Test, and there is a strong possibility that it can be certified. Any Monitor that shows a ※has not run§ status, it may indicate a problem in the monitored component or system. Depending on local regulations, the vehicle may or may not be ready for an Emissions Test.

(NOTE) Some areas require that all Monitors indicate a ※has run§ status before an Emissions Test can be performed. Other areas only require that some, but not all Monitors have run their self-diagnostic testing before an Emissions Test can be performed.

Kb/s 每 Kilobytes-per-second. Used to specify the speed at which data transfers take place.


Live Data 每 ※Real time§ vehicle operational values (volts, rpm, temperature, speed, etc.) generated by sensors, actuators and switches throughout the vehicle. This information can be accessed via the vehicle*s computer and viewed using a Scan Tool. The ※real time§ vehicle operating information (values/status) that the vehicle*s computer supplies to the Scan Tool for each sensor, actuator, switch, etc., is called Parameter Identification (PID) Data.

Long Term Fuel Trim (LTFT) 每 See ※Fuel Trim§

Malfunction Indicator Lamp (MIL) 每 Dashboard-mounted indicator lamp that lights when the vehicle*s on-board computer detects a failure in an emissions-related component or system that causes a Diagnostic Trouble Code (DTC) to be saved to the computer*s memory. The MIL is also referred to as a ※Check Engine§ light.

Manufacturer-Specific DTC 每 See ※Diagnostic Trouble Code (DTC)§

Mb/s 每 Megabytes-per-second. Used to specify the speed at which data transfers take place.

Monitor - Monitors are "diagnostic routines" programmed into the PCM. The PCM utilizes these programs to run diagnostic tests, and to monitor operation of the vehicle's emissions-related components or systems to ensure they are operating correctly and within the vehicle's manufacturer specifications. Currently, up to eleven Monitors are used in OBD 2 systems. Additional Monitors will be added as the OBD 2 system is further developed. Monitor operation is either "Continuous" or "Non-Continuous," depending on the specific monitor.

-          Continuous Monitors 每 Continuous Monitors are designed to continuously monitor their associated components and/or systems for proper operation. Continuous Monitors run constantly when the engine is running.

-          Non-Continuous Monitors - Non-continuous Monitors perform and complete their testing once per trip.

(NOTE) Not all vehicles support all eleven Monitors.

Monitor Has/Has Not Run - The terms "Monitor has run" or "Monitor has not run" are used throughout this manual. "Monitor has run," means the PCM has commanded a particular Monitor to perform the required diagnostic testing on a system to ensure the system is operating correctly (within factory specifications). The term "Monitor has not run" means the PCM has not yet commanded a particular Monitor to perform diagnostic testing on its associated part of the emissions system.

ms - milliseconds


Non-Continuous Monitor 每 See ※Monitors§

OBD1 每 On-Board Diagnostics Version 1 (also referred to as ※OBD I§)

OBD2 每 On-Board Diagnostics Version 2 (also referred to as ※OBD II§)

OBD2 Drive Cycle - An OBD 2 Drive Cycle is an extended set of driving procedures that takes into consideration the various types of driving conditions encountered in real life. These conditions may include starting the vehicle when it is cold, driving the vehicle at a steady speed (cruising), accelerating, etc. An OBD 2 Drive Cycle begins when the ignition key is turned "On" (when cold) and ends when the vehicle has been driven in such a way as to have all the "Enabling Criteria" met for all its applicable Monitors. Only those trips that provide the Enabling Criteria for all Monitors applicable to the vehicle to run and complete their individual diagnostic tests qualify as an OBD 2 Drive Cycle. OBD 2 Drive Cycle requirements vary from one model of vehicle to another. Vehicle manufacturers set these procedures.

On-Board Computer 每 See ※Powertrain Control Module (PCM)§

Parameter Identification (PID) Data 每 See ※Live Data§

Pending Code 每 A DTC recorded on the ※first trip§ for a ※two trip§ code. If the fault that caused the code to be set is not detected on the second trip, the code is automatically erased.

Powertrain Control Module (PCM) - The PCM is the OBD 2 accepted term for the vehicle's "on-board computer." The PCM is the central processing unit in the vehicle*s computer control system. In addition to controlling the engine management and emissions systems, the PCM also participates in controlling the powertrain (transmission) operation. Most PCMs also have the ability to communicate with other computers on the vehicle (ABS, ride control, body etc.).


Short Term Fuel Trim (STFT) 每 See ※Fuel Trim§

Trip - A Trip for a particular Monitor requires that the vehicle is driven in such a way that all the "Enabling Criteria" for the Monitor to run and complete its diagnostic testing are met. The "Trip Drive Cycle" for a particular Monitor begins when the ignition key is turned "On." It is successfully completed when all the "Enabling Criteria" for the Monitor to run and complete its diagnostic testing are met by the time the ignition key is turned "Off." Since each of the eleven monitors is designed to run diagnostics and testing on a different part of the engine or emissions system, the "Trip Drive Cycle" needed for each individual Monitor to run and complete varies.

(NOTE) Do not confuse a "Trip" Drive Cycle with an OBD 2 Drive Cycle. A Trip Drive Cycle provides the "Enabling Criteria" for one specific Monitor to run and complete its diagnostic testing. An OBD 2 Drive Cycle must meet the "Enabling Criteria" for all Monitors on a particular vehicle to run and complete their diagnostic testing.

Warm-up Cycle - Vehicle operation after an engine off period where engine temperature rises at least 40∼F (22∼C) from its temperature before starting, and reaches at least 160∼F (70∼C). The PCM uses warm-up cycles as a counter to automatically erase a specific code and related data from its memory. When no faults related to the original problem are detected within a specified number of warm-up cycles, the code is erased automatically.