US: On Board Diagnostics

US: On Board Diagnostics


Standard type

Standard type: Standards for mandatory OBD systems in light and heavy-duty vehicles

Program name: On Board Diagnostics (OBD)

Goal: Mandatory system requirements for OBD systems – systems which provide self-diagnostic functionality incorporated into the engine control system, in order to alert the vehicle driver/operator about potential problems that can affect the emission performance of the vehicle.

Regulating Body
Conventional pollutant emission limits

Current Standard

All light-duty vehicles and heavy-duty engines sold throughout the United States


On-Board Diagnostic (OBD) systems are integrated into the computers of new vehicles to monitor emissions controls. The first generation of OBD requirements were implemented in California in 1988 and were required on all 1991 and newer vehicles. Today, OBD requirements apply to light-duty vehicles and heavy-duty engines, both in California and under federal EPA requirements.

California OBD Regulation

California OBD requirements for light-duty vehicles and for heavy-duty engines used in vehicles up to 14,000 lbs GVWR (medium-duty vehicles) have been introduced in two steps:

  • OBD I—OBD I was California’s first OBD regulation which required manufacturers to monitor some of the emission control components on vehicles. Required on all 1991 and newer vehicles, OBD I systems were not as effective as possible because they were limited to monitoring only a few of the emission-related components and they were not calibrated to a specific level of emission performance.
  • OBD II—OBD II systems were developed to address the shortcomings of OBD I, and make the system more user-friendly for service technicians. This more rigorous second generation of OBD regulation began phase-in in 1994. Since 1996, its implementation has been required on all new gasoline and alternate fuel passenger cars and trucks sold in California. All 1997 and newer diesel fueled passenger cars and trucks are also required to meet OBD II requirements.

California OBD requirements for heavy-duty engines for vehicles with a GVWR greater than 14,000 lb have also been introduced in two stages, as follows:

  • EMD—Starting with the 2007 model year, heavy-duty engines require a very basic diagnostic system referred to as Engine Manufacturer Diagnostic (EMD) system.
  • HD OBD—Starting in 2010, OBD requirements for heavy-duty engines will be phased-in. By 2013, EMD systems will be phased out and all heavy-duty engines offered for sale in California will also require OBD systems. The HD OBD rule was last amended on 31 July 2013 and includes revisions that accelerate the start date for OBD system implementation on alternate-fueled engines from the 2020 model year to the 2018 model year and relaxes some requirements for OBD systems on heavy-duty hybrid vehicles for the 2013 through 2015 model years. See the updated informative digest for a summary of changes.

Federal OBD Regulation

Following the introduction of OBD requirements in California, OBD regulations were also adopted by the US EPA. The following have been the most important steps in the development of federal OBD requirements:

  • Beginning with the 1994 model year, the EPA has required OBD systems on light-duty vehicles (LDVs) and light-duty trucks (LDTs).
  • Since 2005, OBD systems became mandatory for heavy-duty vehicles and engines up to 14,000 lbs GVWR.
  • In December 2008, EPA finalized OBD regulations for 2010 and later heavy-duty engines used in highway vehicles over 14,000 lbs GVWR and made changes to the OBD requirements for heavy-duty applications up to 14,000 lbs GVWR to align them with requirements for applications over 14,000 lbs GVWR.

Technical Standards

California light-duty and heavy-duty regulations define a number of general requirements for the malfunction indicator light (MIL), trouble codes, monitoring, thresholds and standardized communications common to all OBD systems. These requirements—outlined in the following sections—also apply to systems intended to comply with US federal requirements.

On-board diagnostic capabilities with OBD II systems are incorporated into the hardware and software of a vehicle’s on-board computer for monitoring 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.

MIL & Fault Code Requirements

The Malfunction Indicator Light (MIL) is located on the driver’s side of the instrument panel. Except for a functionality check where it illuminates for 15-20 seconds when in the key-on position before engine cranking, it is normally illuminated only when the OBD system has detected and confirmed a malfunction that could increase emissions.

A number of steps must happen before the MIL illuminates. When the OBD determines that a malfunction has occurred, it generates and stores a “pending fault code” and a “freeze frame” of engine data. At this point, the MIL does not illuminate. If the malfunction is detected again before the next driving cycle in which the suspected system or component is monitored, the MIL illuminates continuously and a “MIL-on” or “confirmed” fault code is generated and stored as well as a “freeze frame” of engine data. If the malfunction is not detected by the end of the driving cycle, the “pending fault code” is erased.

Except for misfires and fuel system faults, if the malfunction is not detected in the next three driving cycles, the MIL can be extinguished but the trouble code is still stored for at least 40 engine warm-up cycles. The MIL can also be extinguished and fault codes erased with a scan tool that technicians use to diagnose malfunctions. Alternate MIL illumination strategies are also possible but subject to approval.


The systems and parameters that require monitoring are outlined below. While some components can be monitored continuously, this is not always possible. Therefore, manufacturers must define conditions under which important emission control components and subsystems can be monitored for proper function. The monitoring conditions should meet the following requirements:

  • Ensure robust detection of malfunctions by avoiding false passes and false indications of malfunctions,
  • Ensure monitoring will occur under conditions that may reasonably be expected to be encountered in normal vehicle operation and use,
  • Ensure monitoring will occur during the FTP cycle.

In order to quantify the frequency of monitoring, an in-use monitor performance ratio is defined as:

In-use monitoring performance ratio = Number of monitoring events / Number of driving events

Each component and subsystem requiring monitoring requires its own ratio. For example, for 2013 and later heavy-duty engines, the minimum acceptable value of this ratio is 0.100 (i.e. monitoring should occur at least during 1 vehicle trip in 10).

Monitoring Requirements of California OBD Systems
System/Component Parameter Requiring Monitoring
Fuel system Fuel system pressure control
Injection quantity
Injection timing
Feedback control
Misfire Detect continuous misfire
Determine % of misfiring cycles per 1000 engine cycles
(2013 and later engines)
EGR (Exhaust Gas Recirculation) Low flow
High flow
Slow response
EGR cooler operation
EGR catalyst performance
Feedback control
Boost pressure Underboost
Slow response
Charge air under cooling
Feedback control
NMHC catalyst Conversion efficiency
Provide DPF heating
Provide SCR feedgas (e.g., NO2)
Provide post DPF NMHC clean-up
Provide ammonia clean-up
Catalyst aging
SCR (Selective catalytic reduction) NOx catalyst Conversion efficiency
SCR reductant:

  • delivery performance,
  • tank level,
  • quality, and
  • injection feedback control
Catalyst aging
NOx adsorber NOx adsorber capability
Desorption function fuel delivery
Feedback control
DPF (diesel particulate filter) Filtering performance
Frequent regeneration
NMHC conversion
Incomplete regeneration
Missing substrate
Active regeneration fuel delivery
Feedback control
Exhaust gas sensors For air-fuel ratio and NOx sensors:

  • performance,
  • circuit faults,
  • feedback, and
  • monitoring capability
Other exhaust gas sensors
Sensor heater function
Sensor heater circuit faults
VVT (variable valve timing) Target error
Slow response
Cooling system Thermostat
ECT sensor circuit faults
ECT sensor circuit out-of-range
ECT sensor circuit rationality faults
CCV (canister closed valve) System integrity
Comprehensive component monitoring
Cold start emission reduction strategy
Other emission control system monitoring

Comprehensive Component Monitoring requires the monitoring of any electronic engine component/system not specifically covered by the regulation that provides input to or receives commands from on-board computers and that can affect emissions during any reasonable in-use driving condition or is used as part of the diagnostic strategy for any other monitored system or component.

Monitoring is also required for all other emission control systems that are not specifically identified. Examples include: hydrocarbon traps, HCCI control systems or swirl control valves.

Malfunction Criteria

Malfunction criteria for the various malfunctions listed above vary depending on the system or component and individual parameter being monitored. In some cases, such as feedback control systems, sensor rationality checks and checks for circuit faults, a go/no-go criteria is used. In other cases such as the fuel system, EGR, turbocharger physical parameters and aftertreatment system performance, the OBD system must be able to determine when deterioration or other changes cause emissions to exceed a specified threshold.

In order to determine malfunction criteria for many of these faults, manufacturers must correlate component and system performance with exhaust emissions to determine when deterioration will cause emissions to exceed a certain threshold. This may require extensive testing and calibration for each engine model.

In determining the malfunction criteria for diesel engine monitors that are required to indicate a malfunction before emissions exceed an emission threshold (e.g., 2.0 times any of the applicable standards), the emission test cycle and standard that would result in higher emissions with the same level malfunction is to be used. Some adjustment is possible for those components experiencing infrequent regeneration.

Manufacturers have the option of simplifying monitoring requirements if failure or deterioration of a parameter will not cause emissions to exceed the threshold limits. For parameters that are controlled, such as temperature, pressure and flow, a malfunction in such a case would only need to be indicated when the commanded setting cannot be achieved. For aftertreatment devices, a malfunction would be indicated when the aftertreatment device has no conversion/filtering capability.

To account for the fact that current technology may not be adequate to detect all malfunctions at the required threshold, some flexibility has been built into the regulations. A manufacturer may request a higher emission threshold for any monitor if the most reliable monitoring method developed requires a higher threshold. Additionally, the PM filter malfunction criteria may be revised to exclude detection of specific failure modes (e.g., partially melted substrates or small cracks) if the most reliable monitoring method developed is unable to detect such failures.

A number of other exceptions are available including the possibility to disable OBD monitoring at ambient engine start temperatures below 20°F or at elevations above 8000 feet above sea level.

Standardization Requirements

OBD systems have a standardization requirement that makes diagnostics possible with a universal scan tool that is available to anyone—not just manufacturer’s repair facilities. The standardization requirements include:

  • A standard data link connector
  • A standard protocol for communications with a scan tool
  • In-use performance ratio tracking and engine run time tracking requirements
  • Engine manufacturers must provide the aftermarket service and repair industry emission-related service information
  • Standardized functions to allow information to be accessed by a universal scan tool. These functions include:
    • Readiness status: The OBD system indicates “complete” or “not complete” for each of the monitored components and systems.
    • Data stream: A number of specific signals are made available through the standardized data link connector. Some of these include: torque and speed related data, temperatures, pressures, fuel system control parameters, fault codes and associated details, air flow, EGR system data, turbocharger data and aftertreatment data.
    • Freeze frame: The values of many of the important parameters available in the Data Stream are stored when a fault is detected.
    • Fault codes
    • Test results: Results of the most recent monitoring of the components and systems and the test limits established for monitoring the respective components and systems are stored and made available through the data link.
    • Software calibration identification: Software Calibration Verification Number.
    • Vehicle Identification Number (VIN)
    • Erasing emission-related diagnostic information: The emission-related diagnostic information can be erased if commanded by a scan tool (generic or enhanced) or if the power to the on-board computer is disconnected.


OBD systems may still be conditionally certified even if they do not comply with one or more of the legal requirements outlined in the regulation and the manufacturer has made a good faith effort at compliance. In such cases, the OBD system is said to contain deficiencies. Deficiencies can only be carried over for up to 3 years.

For 2004 model year passenger cars, light-duty trucks and medium duty vehicles and engines and for 2013 and later heavy-duty engines for vehicles over 14,000 lbs, a fine is imposed on vehicles or engine containing multiple deficiencies sold in California. The amount of the fine depends on the number of deficiencies ($25 or $50/deficiency) and is levied on the third and subsequently identified deficiencies. In some cases for diesel fueled vehicles, the fine is levied on the fourth and subsequent deficiencies. The maximum fine per vehicle or per engine is $500.

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