The On-Board Diagnostic (OBD) system is a crucial component of modern vehicles, constantly monitoring the performance of emission control systems. The Obd Monitor Test is a vital part of vehicle inspections, ensuring that these systems are functioning correctly and vehicles are not excessively polluting. This article provides a comprehensive reference to the OBD monitor test, including pass/fail standards and guidance for handling vehicles with known testing difficulties. This information is essential for automotive technicians, vehicle inspectors, and anyone seeking a deeper understanding of vehicle emission testing.
Readiness Monitor Standards
Readiness monitors are self-diagnostic tests performed by a vehicle’s OBD system to confirm the operational status of emission control components. These monitors run under specific driving conditions, verifying that systems like the catalytic converter, oxygen sensors, and evaporative emissions system are working as intended. While most vehicles complete these monitors during normal driving, some may require specific drive cycles, often outlined in service manuals or OEM technical documents, to set all monitors to a “ready” status. Factors such as driving patterns, component malfunctions, or recent repairs (like battery disconnection) can affect monitor completion.
For Smog Check inspections, the number of incomplete readiness monitors allowed varies depending on the vehicle’s model year and fuel type. The standards are designed to ensure sufficient emission system monitoring while acknowledging that some monitors may not always be immediately ready.
| Model Year | Fuel Type | Number of Incomplete Monitors Allowed to Pass OBD Test¹ |
|---|---|---|
| 1996-1999 | Gas² | Any one³ |
| 2000 and newer | Gas² | Evaporative system³ |
| 1998-2006 | Diesel | Zero |
| 2007 and newer | Diesel | Particulate Filter and Non-Methane HydroCarbon (NMHC)⁴ |
¹ Implemented May 4, 2015.
² “Gas” includes gasoline, propane, natural gas (CNG, LNG, LPG).
³ The misfire, fuel, and comprehensive component continuous monitors are ignored during an OBD test on “gas” vehicles.
⁴ Implemented July 5, 2023.
Readiness monitor standards for OBD tests, detailing the allowable number of incomplete monitors based on vehicle model year and fuel type.
Malfunction Indicator Lamp (MIL) Standards
The Malfunction Indicator Lamp (MIL), often referred to as the “Check Engine” light, is a critical indicator of potential emission control issues. The OBD monitor test includes checks of the MIL’s behavior to identify if there are any active faults or if the MIL system itself is malfunctioning. Specific standards dictate whether a vehicle passes or fails based on the MIL’s status during different engine states.
| Vehicle | Pass/Fail | Standard |
|---|---|---|
| All with OBDII | Pass | MIL illuminates when key on engine off (KOEO). |
| All with OBDII | Pass | MIL turns off when engine running (KOER). |
| All with OBDII | Fail | MIL does not illuminate when key on engine off (KOEO). |
| All with OBDII | Fail | MIL illuminates when engine running (KOER). |
MIL standards for OBD tests, outlining pass and fail criteria based on MIL illumination during key-on engine-off (KOEO) and key-on engine-running (KOER) states.
A properly functioning MIL should illuminate briefly when the ignition is turned to the “key on, engine off” position to indicate it is working. Once the engine is started (“key on, engine running”), the MIL should turn off if no emission faults are detected. Failure to illuminate during KOEO or remaining illuminated during KOER are both indications of a problem that will result in a failed OBD monitor test.
Communication Standards
Effective communication with the vehicle’s OBD system is fundamental to performing an OBD monitor test. The test equipment must be able to establish a connection and retrieve necessary data from the vehicle’s computer. Communication failures can arise from various sources, including issues with the OBD connector, wiring problems, aftermarket installations that interfere with the diagnostic port, or computer system malfunctions within the vehicle itself.
| Vehicle | BAR-97/OIS | Standard |
|---|---|---|
| All with OBDII | BAR-97 | Vehicle communicates with BAR-97. |
| All with OBDII | OIS | Vehicle communicates with OIS and transmits required data.¹ |
¹ Required data typically means data from OBD addresses: Mode $01 PID $00, Mode $01 PID $01. Mode $01 PID $00 returns what data is supported on the vehicle. Mode $01 PID $01 returns MIL status, diagnostic trouble codes (DTCs), and monitor status.
Vehicle communication standards for OBD tests, detailing the requirement for successful communication between the vehicle and testing equipment.
Successful communication means the testing equipment can interface with the vehicle’s OBD system and retrieve essential information, such as supported data parameters, MIL status, diagnostic trouble codes (DTCs), and readiness monitor statuses. Failure to establish this communication will lead to an inconclusive or failed OBD monitor test, requiring further investigation into the cause of the communication issue.
Permanent Diagnostic Trouble Code (PDTC) Standards
Permanent Diagnostic Trouble Codes (PDTCs) are a specific type of DTC that, unlike regular DTCs, cannot be cleared using a scan tool or by disconnecting the vehicle’s battery. PDTCs are stored when the OBD system detects an emission-related fault. They are designed to ensure that a vehicle has truly addressed and resolved an issue before passing an emission test. PDTCs will only clear themselves once the OBD system verifies, through subsequent drive cycles, that the fault is no longer present.
Since July 1, 2019, PDTC checks have become a part of Smog Check inspections in some regions. Vehicles are subject to failure if PDTCs are present, regardless of whether the MIL is currently illuminated.
| Model Year | BAR-97/OIS | Standard |
|---|---|---|
| 1996-1999 | BAR-97 | Not applicable. |
| 2000-2009 | OIS | Not applicable. |
| 2010 and newer ¹ | OIS | Vehicles fail when a PDTC is present. ²,³,⁴,⁵ |
¹ Some 2010 vehicles do not support PDTC. Those without support for PDTC will not be subject to the PDTC check.
² A vehicle with a PDTC stored will fail the PDTC check regardless of whether the MIL is commanded ON or OFF.
³ PDTC presence will be ignored if the vehicle has completed at least 15 warm-up cycles and been driven at least 200 miles since its OBD information was last cleared (as determined by Mode $01 PID $30 and $31).
⁴ BAR is aware of some vehicles that have trouble properly clearing certain PDTCs. BAR is working with the California Air Resources Board (CARB) to ensure that known problematic vehicles are addressed by vehicle manufacturers. Until then, these codes will be ignored by BAR-OIS. For more information, see Permanent Diagnostic Trouble Code Vehicles of Interest.
⁵ Vehicle manufacturers have issued extended warranties, recalls and repair campaigns for some vehicles that have trouble properly clearing PDTCs. These vehicles are listed under Permanent Diagnostic Trouble Code Vehicles of Interest. When one of these vehicles fails for the PDTC, please refer the motorist to the dealer for covered repairs.
PDTC standards for OBD tests, specifying the model years subject to PDTC checks and the conditions under which vehicles will fail due to the presence of PDTCs.
It’s important to note that some vehicles are known to have issues with clearing certain PDTCs. Inspection systems may be programmed to ignore specific PDTCs on these vehicles, and manufacturers sometimes issue recalls or extended warranties to address these problems. If a vehicle fails due to a PDTC, particularly if it’s a known issue vehicle, it’s advisable to consult manufacturer resources or refer the vehicle to a dealership for potential repairs covered under warranty or recall.
Modified Software Standards
To ensure the integrity of emission control systems, checks for modified or tampered software were introduced to Smog Check inspections on July 19, 2021. Vehicles with software that has been illegally modified to bypass or defeat emission controls will fail the OBD monitor test. Software that has been approved by regulatory bodies like the California Air Resources Board (CARB) through an Executive Order is permissible and should not cause a failure.
If modified software is detected, the vehicle will fail the inspection, and the vehicle inspection report will indicate “Modified Software.” To pass a subsequent inspection, the vehicle must be restored to an OEM-approved or CARB-approved software configuration. Retesting can then be performed at a licensed Smog Check station, unless directed otherwise by inspection authorities.
Vehicles of Interest: Addressing OBD Test Challenges
Certain vehicle makes and models are known to exhibit specific issues during OBD monitor tests. These issues can range from communication problems and incomplete readiness monitors to false MIL illumination or OBD test incompatibility. Inspection systems are often programmed to accommodate some of these known design anomalies. For other cases, vehicle repair or software updates are necessary to resolve the underlying issues.
Sections in the original document detail specific “Vehicles of Interest” for both OIS (Older Inspection System) and BAR-97 EIS (Enhanced Inspection System) inspections. These sections provide valuable information for inspectors and technicians, listing vehicle makes, models, and years that may require special handling or have known OBD testing challenges. The tables in the original document outline the specific issue, the cause and possible remedies, and instructions for inspectors to handle these situations. For detailed information on specific vehicles and potential OBD test issues, please refer to the original linked document.
Conclusion
The OBD monitor test is a critical component of vehicle emission inspections, verifying the proper function of essential emission control systems. Understanding the pass/fail standards for readiness monitors, MIL function, communication, PDTCs, and software modifications is crucial for accurate and effective vehicle inspections. Furthermore, awareness of vehicle-specific issues and known OBD testing challenges ensures that inspections are conducted efficiently and fairly, contributing to cleaner air and a healthier environment. Regular OBD monitor testing and addressing any identified issues are essential for maintaining vehicle emission compliance and optimal performance.
