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.

Monitoring

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).

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.

Deficiencies

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.