OBD II is an acronym for On-Board Diagnostic II, the second generation of on-board self-diagnostic equipment requirements for light- and medium-duty California vehicles.Checked 2026-08-12
OBD-II (On-Board Diagnostics II)
The standardised self-diagnostic system in modern cars, read through a connector under the dash.
Also called: OBD2 · OBD-2 · on-board diagnostics
What it means
Definition
CARB describes OBD II as the second generation of on-board self-diagnostic equipment requirements for light- and medium-duty California vehicles, and states that all 1996 and newer model year gasoline and alternate fuel passenger cars and trucks are required to have OBD II systems. The system watches emissions-related components, stores a code when it finds a fault, and lights the dashboard lamp. Because the connector and the generic codes are standardised, any scan tool can read them.
Why a buyer cares
What it changes
It is the one part of a used car that will answer questions from a stranger. A scan takes minutes and does not depend on the seller's account of the car.
In practice
Where you meet it
The 16-pin connector under the dashboard near the steering column, and every scan-tool or emissions-test printout.
Two things that look alike
Not the same as
OBD-I systems, before standardisation, differed by manufacturer and generally needed the maker's own equipment.
Read next
Related terms
Diagnostic Trouble Code (DTC)
The code an OBD-II system stores to identify which monitored system it found a fault in.
Readiness monitors
Self-tests the OBD-II system must complete before an emissions test can be read.
Malfunction Indicator Lamp (MIL)
The dashboard lamp — usually labelled Check Engine — that the OBD-II system switches on when it confirms a fault.
Emissions inspection
A state or local test of a vehicle's emissions performance, often required to register it.
Where this comes from
Sources
All 1996 and newer model year gasoline and alternate fuel passenger cars and trucks are required to have OBD II systems.Checked 2026-08-12
Both quotes are CARB's and framed around California vehicles. No equally plain EPA sentence stating the 1996 model-year line nationally was found.