OBD II is an acronym for On-Board Diagnostics II, the second generation of on-board self-diagnostic strategy. On-board diagnostic capabilities are incorporated into the hardware and software of a vehicle's on-board computer to monitor virtually every component that can affect emission performance. Each component is checked by a diagnostic routine to verify that it is functioning properly. If a problem or malfunction is detected, the OBD II system illuminates a warning light on the vehicle instrument panel to alert the driver. This warning light will typically display the phrase "Check Engine" or "Service Engine Soon." The system will also store important information about the detected malfunction so that a repair technician can accurately find and fix the problem. Required on 1996 and later vehicles sold in the US.

OBD-II terminology consists of acronyms, jargon, shorthand and code phrases that have meaning to the select few and sometimes seem for the rest of us. Here is a chance to untangling these symbols.
AFC - Air Flow Control
ALDL - Assembly Line Diagnostic Link. Former name for GM (only) Data Link Connector, the connector socket into which the scan tool plug is inserted; sometimes used to refer to any pre-OBD II computer signals
CAN - Controller Area Network
CARB - California Air Resources Board
CFI - Central Fuel Injection (a.k.a. Throttle Body Fuel Injection TBI)
CFI - Continuous Fuel Injection
CO - Carbon Monoxide
DLC - Data Link Connector
Driving Cycle - A specific sequence of start-up, warm-up and driving tasks that tests all OBD II functions (Details)
DTC - Diagnostic Trouble Code
ECM - Engine Control Module- usually the main in-car computer controlling emissions and engine operation
ECT - Engine Coolant Temperature
EEC - Electronic Engine Control
EEPROM or E2PROM - Electrically Erasable Programmable Read Only Memory
EFI - Electronic Fuel Injection
EGR - Exhaust Gas Recirculation
EMR - Electronic Module Retard
EPA - Environmental Protection Agency. Federal agency. Office of Mobile Sources is the branch concerned with auto emissions.
ESC - Electronic Spark Control
EST - Electronic Spark Timing
DPFE - Differential Pressure Feedback EGR sensor ( On Ford OBDII systems).
DTC - Diagnostic Trouble Code
FLI - Fuel Level Indicator
Fuel Trim - Engine computer function that keeps the air/fuel mixture as close to the ideal 14.7:1 stoichiometric ratio as possible
HC - Hydrocarbons
HEI - High Energy Ignition
HO2S - Heated Oxygen Sensor
IAT - Intake Air Temperature
ISO 9141 - International Standards Organization OBDII communication mode, used by Chrysler and most foreign cars. One of three hardware layers defined by OBD II
J1850PWM - (Pulse Width Modulated) SAE-established OBD II communication standard used by Ford domestic cars and light trucks. One of three hardware layers defined by OBD II
J1850VPW - (Variable Pulse Width Modulated) SAE-established OBD II communication standard used by GM cars and light trucks. One of three hardware layers defined by OBD II
J1962 - SAE-established standard for the connector plug layout used for all OBD II scan tools
J1978 - SAE-established standard for OBD II scan tools
J1979 - SAE-established standard for diagnostic test modes
J2012 - SAE-established standard accepted by EPA as the standard test report language for emission tests
MAF - Mass Air Flow
MAP - Manifold Absolute Pressure
MAT - Manifold Air Temperature
MFG - Manufacturer
MIL - Malfunction Indicator Light. The "Check Engine Light" on your dash.
NOx - Oxides of Nitrogen
O2 - Oxygen
OBD - On-Board Diagnostics
OBDII or OBD II - Updated On-Board Diagnostics standard effective in cars sold in the US after 1-1-96
Parameters - Readings on scan tools representing functions measured by OBD II and proprietary readings
PCM - Powertrain Control Module, the on-board computer that controls engine and drive train
PCV - Positive Crankcase Ventilation
PID - Parameter ID
Proprietary Readings - Parameters shown by on-board computers which are not required by OBD II, but included by manufacturer to assist in trouble-shooting specific vehicles.
PTC - Pending Trouble Code
RPM - Revolutions Per Minute
SAE - Society of Automotive Engineers, professional organization that set the standards that EPA adopted for OBD and OBD II
Scan Tool - Computer based read-out equipment to display OBD II parameters
SES - Service Engine Soon dash light, now referred to as MIL
SFI - Sequential Fuel Injection
Stoichiometric Ratio - Theoretical perfect combustion ratio of 1 part gas to 14.7 parts air
TBI - Throttle Body Injection
TPI - Tuned Port Injection
TPS - Throttle Position Sensor
VAC - Vacuum
VCM - Vehicle Control Module, the in-car computer that oversees engine management, transmission operation, anti-lock brakes, and other functions not directly related to emissions control
VIN - Vehicle Identification Number
VSS - Vehicle Speed Sensor
WOT - Wide Open Throttle