OBD II Monitors
The California Air Resources Board (CARB) began regulating On Board Diagnostic
(OBD) systems for vehicles sold in California beginning with the 1988 model year.
The initial requirements, known as OBD I, required identifying the likely area of
malfunction with regard to the fuel metering system, Exhaust Gas Recirculation (EGR)
system, emission-related components and the Powertrain Control Module (PCM). A malfunction
indicator lamp (MIL) labeled CHECK ENGINE or SERVICE ENGINE SOON was required to illuminate
and alert the driver of the malfunction and the need to service the emission control system.
A fault code or Diagnostic Trouble Code (DTC) was required to assist in identifying the
system or component associated with the fault.
Starting with the 1994 model year, both CARB and Environmental Protection
Agency (EPA) mandated enhanced OBD systems, commonly known as OBD-II. The
objectives of the OBD-II system are to improve air quality by reducing high
in-use emissions caused by emission-related malfunctions, reducing the time
between the occurrence of a malfunction and its detection and repair, and
assisting in the diagnosis and repair of emission-related problems. By the
1996 model year, all California passenger cars and trucks (up to 14,000 lb
GVWR) and all federal passenger cars and trucks (up to 8,5000 lb GVWR) are
required to comply with either CARB-OBD II or EPA OBD requirements. These
requirements apply to gasoline vehicles, diesel vehicles and are being
phased in on alternative-fuel vehicles as well.
The OBD II system monitors virtually all emission control systems and
components that can affect tailpipe or evaporative emissions. In most cases,
malfunctions must be detected before emissions exceed 1.5 times the
applicable 50K- or 100K-mile emission standards. If a system or component
exceeds emission thresholds or fails to operate within a manufacturer's
specifications, a DTC will be stored and the MIL will be illuminated within
two driving cycles.
The OBD II system monitors for malfunctions either continuously, regardless
of driving mode, or non-continuously, once per drive cycle during specific
drive modes. A pending DTC is stored in the PCM Keep Alive Memory (KAM) when
a malfunction is initially detected. This pending DTC maybe erased on the
third vehicle restart after two consecutive drives cycles with no
malfunction. However if the malfunction is still present after two
consecutive drive cycles, the MIL is illuminated. Once the MIL is
illuminated, three consecutive drive cycles without a malfunction detected
are required to extinguish the MIL. The DTC is erased after 40 engine
warm-up cycles once the MIL is extinguished.
In addition to specifying and standardizing much of the diagnostics and MIL
operation, OBD-II requires the use of a standard Diagnostic Link Connector
(DLC), standard communication links and messages, standardized DTCs and
terminology. Examples of standard diagnostic information are freeze frame
data and Inspection Maintenance (IM) Readiness Indicators.
Freeze frame data describes data stored in KAM at the point the malfunction
is initially detected. Freeze frame data consists of parameters such as
engine rpm and load, state of fuel control, spark, and warm-up status.
Freeze frame data is stored at the time the first malfunction is detected,
however, previously stored conditions will be replaced if a fuel or misfire
fault is detected. This data is accessible with the scan tool to assist in
repairing the vehicle.