Active Testing

Diagnostic testing in which the PCM controls the system or component in a specific action while monitoring takes place.


An abbreviation for Secondary Air Injection. An emissions system found primarily on large-engine vehicles that pumps fresh air into the exhaust stream to reduce HC and CO emissions.

Baud Rate

The speed at which bits of computer information are transmitted on a serial data stream. Measured in bits per second (bps).


The individual voltage signal of a serial data stream; also, the smallest unit of measurement recognized by a computer. Refers to a digital data transfer rate. Class 2 data has a 10.4 kilobits per second transfer rate.

Class 2

The type of digital data stream utilized in most GM OBD II diagnostic systems. Faster than UART, the data stream used on non-OBD II diagnostic systems. Class 2 uses two-bit pulse widths and toggles between 0 (passive) and 7 volts (active).


An acronym for Carbon Monoxide, a colorless, odorless and highly poisonous gas. It is formed by incomplete combustion of gasoline.


Any number of on-board tests run by the Diagnostic Management System, which checks for malfunctions, errors or breakdowns in vehicle systems or components.

Diagnostic Executive

The Diagnostic Management System software that stores testing results and controls the illumination
of the lamps.

Diagnostic Management System

The PCM system responsible for performing testing of powertrain components and systems, recording testing results and performing TEST FAIL actions.


An acronym for Driver Information Center. It displays messages to alert the driver of malfunctions, warnings and instructions. The DIC may display alerts related to type C diagnostic trouble codes.


An acronym for Data Link Connector. Formerly referred to as the ALDL, this is the connector to which diagnostic scan tools are connected. Under OBD II, the DLC is a standardized 16-cavity connector and has a standardized location under the driver-side instrument panel.


An acronym for Diagnostic Trouble Code. It is also referred to as a fault code or code. Any code stored in the PCM memory.


An acronym for Engine Control Module, the on-board computer that controls fuel and emissions, as well as diagnostics, for the vehicle's engine management system.


An acronym for Engine Coolant Temperature sensor.


An acronym for Exhaust Gas Recirculation. An emissions system which recirculates some of the exhaust gas back into the intake manifold to reduce NOx (oxides of nitrogen) emissions. Under full OBD II implementation, the PCM will monitor the EGR system for effectiveness.


Gases and particles left over after the combustion event of an engine, or from a fuel system. The primary emissions of concern are hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx).

Enable Criteria

These are the exact conditions required for a diagnostic test to run.


An acronym for the Environmental Protection Agency, the federal government agency that establishes regulations and oversees the enforcement of laws related to the environment. Included in these laws are regulations on the amount and content of automotive emissions.


An abbreviation for Evaporative Emission System. This system is used to collect fuel vapor from the fuel tank. The fuel vapors are drawn into the intake manifold and combusted during certain engine operating conditions.

Fail Record

These records store information about operating conditions when a DTC is stored. The PCM has the ability to store multiple Fail Records, and it also has the ability to update Fail Records.

Freeze Frame

Operating conditions which are stored in the memory of the PCM at the instant a DTC is stored and the MIL is illuminated. A Freeze Frame is stored for only one DTC and can only be overwritten under certain conditions.


An acronym for Federal Test Procedure. A stringent series of automotive tests which the EPA uses to measure and certify the emissions output of all fleets of new cars and light-duty trucks sold in the United States.

Fuel Trim

An ECM function that adjusts fuel delivery during closed-loop operation to bring the air/fuel mix to as close to the optimum ratio (14.7:1) as possible.


An acronym for Hydrocarbons. Any number of compounds of carbon and hydrogen used as fuel, such as gasoline. High levels of hydrocarbons in tailpipe emissions are a result of unburned gasoline.


An abbreviation for Heated Oxygen Sensor.


An acronym for Idle Air Control valve.


An acronym for Intake Air Temperature sensor.


An abbreviation for Inspection and Maintenance, usually referring to state emissions inspection and testing programs.

Intrusive Diagnostic Test

Any on-board test run by the PCM which could have an effect on vehicle performance or emissions.

ISO 9141

International Standards Organization recommended data communication network interface.


An acronym for Mass AirFlow sensor.


An acronym for Manifold Absolute Pressure.


An acronym for Malfunction Indicator Lamp. The MIL was formerly called the "service engine soon" or "check engine" lamp.


When incomplete or no combustion occurs in one or more cylinders due to improper fuel, ignition, cylinder compression, or air.


An acronym for Oxides of Nitrogen, a primary emission produced in the combustion chamber under high temperatures when nitrogen combines with oxygen. Oxides of nitrogen contribute to the formation of smog (O3).


An acronym for On-Board Diagnostics Generation One. An on-board automotive diagnostic system required by the California Air Resources Board since 1988, which uses a microprocessor and sensors to monitor and control various engine drivability functions.


An acronym for On-Board Diagnostics Generation Two. OBD II expands upon OBD I to include emissions system and sensor deterioration monitoring.

Passive testing

During this type of testing, the PCM monitors the component or system during normal operation.


An acronym for Powertrain Control Module, the on-board control module that monitors both engine and transmission/transaxle functions.


An acronym for the Society of Automotive Engineers, a professional organization made up of automotive engineers and designers that establishes standards and conducts testing for many automotive-related functions.

Statistical Filtering

The PCM internally charts the results of diagnostic testing over a period of time and creates a baseline for testing. By doing this, the PCM is able to filter out information that could cause a false DTC to set.

System Status (I/M Ready)

A signal for emission testing which states that all the vehicle's on-board emissions diagnostics have been run. System status (I/M ready) is not concerned whether the emission system passed or failed the test, only that it was run.


An acronym for Torque Converter Clutch. A clutch device found in automatic transaxles or transmissions, which creates a fluid coupling between the engine and the final drive output.


An acronym for Throttle Position sensor.


A key cycle (key ON, run, key OFF/power-down) where the enable criteria for a particular diagnostic are met and the diagnostic test runs.


An acronym for Three-Way Converter.


Universal Asynchronous Receive and Transmit. The type of data stream used on non-OBD II diagnostic systems. UART toggles between 5 volts (passive) and 0 volts (active).


An acronym for Vehicle Control Module, the on-board computer that controls the engine management, transmission, and other systems such as antilock brakes.

Warm-Up Cycle

The PCM uses warm-up cycles instead of key cycles to clear DTCs. A Warm-Up Cycle is achieved when the engine coolant temperature rises at least 22 C (40 F) from start-up and achieves a minimum temperature of 71 C (160 F).


When do I change my oil

Freon changeover

Why did I fail my emission test

Wiring Diagrams 

TSB's and Recalls



Timing Belts_ 1970-1997 Domestic & Imported Cars

Brake Squeals


Oxygen Sensors

Please feel free to contact me at the following address:

 E-mail: stberry@netzero.net



Last Updated on 4/20/2006
by Scott Throneberry