OBD2 glossary
Short definitions of the terms you see in a scanner app and in our code guides. The related guides linked below explain each topic in more detail.
Terms
- OBD2 (OBD II)
- On-Board Diagnostics, second generation. The self diagnostic system required on US gasoline cars and light trucks from the 1996 model year. It monitors emission related components, turns on a warning light and stores trouble codes.
- EOBD
- European On-Board Diagnostics, the EU equivalent of OBD2, required on new petrol cars from 2001 and new diesel cars from 2004 (registration dates).
- DLC (data link connector)
- The standard 16 pin OBD2 port, generally under the dashboard on the driver's side.
- MIL (malfunction indicator lamp)
- The check engine light. It comes on for a confirmed emission related fault and blinks for a misfire that could damage the catalytic converter.
- DTC (diagnostic trouble code)
- A five character code such as P0171 that identifies which self test failed.
- Generic code
- A code whose second character is 0 (or a P2 code). Its title is standardized by SAE J2012 and is meant to be the same on every OBD2 vehicle.
- Manufacturer specific code
- A code whose second character is 1. Its meaning depends on the carmaker.
- Pending code
- A fault detected once. It becomes a confirmed code if it is detected again on the next drive cycle, and does not turn on the light by itself.
- Permanent code (PDTC)
- A copy of a confirmed code that cannot be erased with a scan tool or by disconnecting the battery. The car clears it after verifying the repair.
- Freeze frame
- A snapshot of engine conditions, such as RPM, coolant temperature, load and vehicle speed, saved when an emission related code is stored.
- Readiness monitor
- One of the OBD system's emission self tests. It reports ready, not ready or not supported.
- Continuous monitor
- A monitor that runs all the time with the engine running: misfire, fuel system and comprehensive components.
- Drive cycle
- The set of driving conditions a car needs so that its non-continuous monitors can run. It varies by manufacturer.
- CAN (ISO 15765-4)
- The OBD2 communication protocol used by most modern cars, and required on cars sold in the US from the 2008 model year.
- ELM327
- An OBD interpreter chip from Elm Electronics that supports all five OBD2 protocols. Many phone adapters use it or a clone of it.
- ELM329
- A CAN only version of the ELM327 chip.
- EVAP
- The evaporative emission control system, which stores fuel vapor and burns it in the engine instead of letting it escape.
- EGR
- Exhaust gas recirculation, which feeds a small amount of exhaust back into the intake to reduce emissions.
Where to learn more
For code structure see How to read an OBD2 code. For code types and freeze frame see Stored, pending and permanent codes. For inspection rules see OBD2 readiness monitors explained.
Try it on your own car
OBD Master Pro turns your Android phone and a generic ELM327 adapter into a code reader: read and clear codes, view freeze frame and live data, and check emissions readiness.
Keep reading
Related guides: Check engine light on: what to do · How to read an OBD2 code · Stored, pending and permanent codes · OBD2 readiness monitors explained · ELM327 adapter not connecting · How to clear the check engine light · What is fuel trim · OBD2 live data and PIDs · OBD2 port pinout · What is a U code?
Common codes: P0171 System Too Lean · P0420 Catalyst Efficiency · P0300 Random Misfire · P0455 EVAP Large Leak
All OBD2 guides · OBD2 by car brand · OBD2 trouble code guides
Sources
- California Air Resources Board (CARB): On-Board Diagnostic II (OBD II) Systems fact sheet
- Clemson University Vehicular Electronics Laboratory: On-Board Diagnostic Systems
- U.S. Code of Federal Regulations, 40 CFR 86.1806-05, On-board diagnostics (govinfo.gov)
- U.S. EPA: Performing Onboard Diagnostic System Checks as Part of a Vehicle Inspection and Maintenance Program (EPA420-R-01-015)
- SAE J2012: Diagnostic Trouble Code Definitions
- Maryland Department of the Environment, Vehicle Emissions Inspection Program: Decoding DTCs
- Ford service information: On Board Diagnostics II PCED, Quick Test and Continuous Memory (pending codes)
- Ford service information: On Board Diagnostics II PCED, Freeze Frame Data
- California Bureau of Automotive Repair: On-Board Diagnostic Test Reference
- Texas Department of Public Safety: Ready or Not (OBD II readiness monitors)
- Elm Electronics (maker of the ELM327 chip): OBD interpreter ICs and protocol support
- Federal Register 68 FR 35792 (June 17, 2003): EPA rule making ISO 15765-4 (CAN) the only OBD protocol from the 2008 model year
- EUR-Lex: Directive 98/69/EC (EOBD requirements and dates for passenger cars)
Published . Updated . General information only. Details generally vary by model, year and market, so always follow the owner's manual and service information for your vehicle.