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On-Board Diagnostics


On-board diagnostic (OBD) systems 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. OBD requirements were first introduced for light-duty vehicles in California in 1991. Today, OBD requirements apply to light-duty vehicles and heavy-duty engines, both in California and under the federal EPA requirements.

The most detailed requirements for OBD systems are provided by the California regulations. Because systems developed for use in California can generally be used for compliance with EPA requirements with only minor differences, it is expected that OBD systems for vehicles and engines sold outside of California will be similar.

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. Detailed OBD requirements, such as OBD thresholds and timing, are covered in separate articles:

MIL & Fault Code Requirements

The Malfunction Indicator Light (MIL) is located on 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 things 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 3 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 in Table 1. 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).

Table 1
Monitoring Requirements of California OBD Systems
System/ComponentParameter Requiring Monitoring
Fuel systemFuel system pressure control
Injection quantity
Injection timing
Feedback control
MisfireDetect continuous misfire
Determine % of misfiring cycles per 1000 engine cycles
(2013 and later engines)
EGRLow flow
High flow
Slow response
EGR cooler operation
EGR catalyst performance
Feedback control
Boost pressureUnderboost
Slow response
Charge air under cooling
Feedback control
NMHC catalystConversion efficiency
Provide DPF heating
Provide SCR feedgas (e.g., NO2)
Provide post DPF NMHC clean-up
Provide ammonia clean-up
Catalyst aging
SCR NOx catalystConversion efficiency
SCR reductant:
  • delivery performance,
  • tank level,
  • quality, and
  • injection feedback control
Catalyst aging
NOx adsorberNOx adsorber capability
Desorption function fuel delivery
Feedback control
DPFFiltering performance
Frequent regeneration
NMHC conversion
Incomplete regeneration
Missing substrate
Active regeneration fuel delivery
Feedback control
Exhaust gas sensorsFor air-fuel ratio and NOx sensors:
  • performance,
  • circuit faults,
  • feedback, and
  • monitoring capability
Other exhaust gas sensors
Sensor heater function
Sensor heater circuit faults
VVTTarget error
Slow response
Cooling systemThermostat
ECT sensor circuit faults
ECT sensor circuit out-of-range
ECT sensor circuit rationality faults
CCVSystem 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 in Table 1 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.