On-Board Diagnostic (OBD) systems are essential self-monitoring systems within vehicle computers. These systems are designed to oversee virtually every component that could potentially affect vehicle emissions. When an issue arises with an emissions-related component, the OBD system activates a warning light on the dashboard, alerting the driver. Simultaneously, it stores detailed information about the detected malfunctions, which is invaluable for facilitating necessary repairs.
For OBD systems to effectively perform their monitoring and fault detection of emission-related problems, vehicles must be operated for a sufficient period. This operation allows the system to complete a comprehensive diagnostic monitoring cycle for each component under its purview. Once this cycle is complete, the OBD system achieves a “ready” state. In this state, it can accurately assess whether a vehicle has any performance issues related to emissions. If a vehicle’s OBD system has been recently reset or cleared – often referred to as “code clearing” – it requires time and vehicle operation to return to a fully ready state. OBD system resets can occur due to battery disconnection or when a scan tool is used to clear codes.
The Clean Truck Check program’s OBD readiness testing introduces a new requirement for some vehicle owners. Recognizing this, the program is being implemented with a phased approach to OBD readiness requirements as it matures. Initially, the program will set minimum operational thresholds that will gradually become more stringent until full readiness criteria are enforced.
Current OBD Testing Readiness Criteria
Currently, the minimum operational requirement for a Clean Truck Check test submission is that the vehicle must have completed at least five (5) Warm-Up Cycles (WUCs) since the codes were last cleared.
An OBD test submitted for the Clean Truck Check that fails to meet this minimum operation threshold will receive a “Not Ready” (failing) result. Consequently, it will not satisfy the emissions compliance testing requirements.
Clean Truck Check Background
Vehicles Subject to OBD Testing
- Heavy-duty diesel vehicles and diesel hybrids with a Gross Vehicle Weight Rating (GVWR) of 14,000 lbs. or more, equipped with 2013 and newer model year engines.
- Heavy-duty alternative fuel vehicles (e.g., natural gas) and alternative fuel hybrids with 2018 and newer model year engines.
Purpose of the OBD System
- To monitor the efficiency of Emission Control Systems (ECS).
- To identify ECS components that require maintenance or repair.
- To alert drivers when emission-related problems occur.
- To assist technicians in diagnosing and repairing emission issues.
- To encourage the development of robust and durable ECS.
Getting Ready for the Clean Truck Check OBD Test
Steps to Maximize Your Chances of Passing the Clean Truck Check OBD Test:
- Address the Malfunction Indicator Light (MIL): If your MIL is illuminated, it indicates an emissions problem. Get your vehicle diagnosed and repaired as soon as possible.
- Test Early: Conduct the OBD test early within your compliance window. This allows ample time to address any potential issues and re-test your vehicle before the compliance deadline if necessary.
- Avoid Clearing Codes: Clearing diagnostic trouble codes (DTCs) resets the OBD system’s readiness status. The OBD system needs sufficient vehicle operation to complete its emission control system monitor checks. Vehicles without enough operation since the last code clear are likely to fail the compliance test. Code clearing can inadvertently happen when:
- Disconnecting the vehicle battery.
- Using an improperly installed kill switch that cuts power to the OBD system (kill switches that only affect accessories are acceptable).
- Using an OBD scan tool to intentionally or unintentionally clear codes.
- Verify Test Results: Ensure your OBD test results are promptly recorded in your CTC-VIS account after testing. If results do not appear within five minutes, you might need to:
- Re-test the vehicle.
- Disconnect any other devices plugged into the diagnostic port that might interfere with the test.
- Check the communication link between the vehicle and the OBD test device to ensure proper data transfer.
- Confirm a stable internet connection for data upload.
- Seek Support: If results remain unavailable after multiple attempts, contact the program support team at [email protected] for assistance.
Core Elements of the Clean Truck Check OBD Test
The Clean Truck Check OBD test assesses several key elements to determine compliance:
- Malfunction Indicator Light (MIL) Status and Diagnostic Trouble Codes (DTCs)
- Monitor Readiness
- Operation Since Codes Cleared (OSC)
- Permanent Diagnostic Trouble Codes (PDTCs)
MIL Status
The OBD scan tool communicates with the vehicle’s computer to read Diagnostic Trouble Codes (DTCs). If a DTC is actively causing the MIL to be on, it signifies an ongoing emissions-related problem. In such cases, the vehicle will fail the Clean Truck Check.
Monitor Readiness
The OBD system maintains readiness monitors that indicate whether the system is “ready” for an emissions compliance test. A vehicle is considered “not ready” and will fail if it has not been operated sufficiently since the last OBD memory clearing (code clear). This lack of operation means the system hasn’t had enough time to complete a full diagnostic assessment to confirm emissions system functionality.
A code clear might occur after a fault repair or if the vehicle has experienced a battery issue (dead, disconnected, or replaced). It doesn’t inherently mean there is a current problem, but rather that the vehicle needs more drive time to run its self-diagnostics and confirm readiness for testing.
Vehicle OBD systems monitor numerous components and operations, grouped into various “Monitor groups.” The following table provides a general overview of typical Monitor groups. The specific monitors within each group can vary based on the vehicle manufacturer, engine type, fuel, model year, and communication protocol.
| Typical Readiness Monitor Groups for Diesel Vehicles | Typical Readiness Monitor Groups for HD Alternative Fuel Vehicles |
|---|---|
| Misfire | Misfire |
| Fuel System | Fuel System |
| Comprehensive Components | Comprehensive Components |
| Diesel Oxidation Catalyst | Catalyst |
| NOx/Selective Catalyst Reduction Aftertreatment | Heated Catalyst |
| Boost Pressure | Evaporative System |
| Exhaust Gas Sensor | Secondary Air |
| Particulate Matter Filter | Oxygen Sensor |
| Exhaust Gas Recirculation / Variable Value Timing | Heated Oxygen Sensor |
| Exhaust Gas Recirculation/Variable Valve Timing |
Data from Clean Truck Check tests indicate that most vehicles have all Monitors in a ready state when tested for compliance. However, vehicles that have recently had codes cleared are likely to show multiple Monitors as “not ready” or “incomplete.”
Operation Since Codes Cleared (OSC)
OBD systems track vehicle operation data since the last DTC clearing. Common OSC metrics include:
- Warm-Up Cycles (WUC) since cleared
- Distance traveled (in kilometers) since cleared
- Engine run time (in minutes) since cleared
Most vehicles in the Clean Truck Check program track WUCs. If a vehicle does not track WUCs, a CARB-certified OBD scan tool will check for distance traveled, and if that is unavailable, engine run time. Readiness criteria will be applied to only one of these OSC variables. Vehicles not meeting the minimum OSC threshold for the tracked variable will receive a “Not Ready” test result. These criteria thresholds will become stricter over time as the program is implemented.
Vehicle Operation Required for Monitor Readiness
Heavy-duty diesel vehicles generally require considerable operation to achieve OBD readiness. Data from the Clean Truck Check shows that achieving full Monitor readiness can require dozens of Warm-up Cycles, thousands of miles, and hundreds of hours of engine run time. This highlights the importance of proactive maintenance and avoiding unnecessary code clearing.
Permanent Diagnostic Trouble Codes (PDTCs)
What are Permanent Diagnostic Trouble Codes?
Permanent Diagnostic Trouble Codes (PDTCs) are similar to regular DTCs but with a critical difference: they cannot be cleared by simply disconnecting the battery or using a standard OBD scan tool. PDTCs can only be cleared by addressing the underlying issue that triggered them and then allowing the vehicle sufficient drive time to re-run the relevant monitor. Once the monitor runs successfully without detecting the fault, the PDTC will clear itself automatically.
Why PDTCs are Important for Clean Truck Check
The inability to easily clear PDTCs is crucial to prevent circumvention of emissions testing. Techniques like disconnecting the battery or using scan tools to clear codes have been used to temporarily mask emission malfunctions and pass inspections. By including PDTCs in the Clean Truck Check, the program reduces the chance of vehicles with unresolved emissions issues from passing, which helps maintain air quality and program effectiveness. While readiness indicators already minimize the chances of passing with active DTCs, PDTCs add an extra layer of assurance that emission control systems are functioning correctly.
PDTCs in Clean Truck Check Compliance Testing
When implemented, vehicles with stored PDTCs in their OBD system will automatically fail the Clean Truck Check OBD test, regardless of whether the MIL is currently illuminated. The presence of a PDTC indicates that the OBD system has not yet confirmed that a previously detected emissions-related fault is no longer active.
Exceptions for PDTC-Related Failures
Yes, there are exceptions. PDTCs will be disregarded if the vehicle has accumulated a reasonable amount of operation since its OBD information was last cleared. OSC criteria specific to PDTCs (separate from general OSC criteria) are being phased in, starting in September 2024 at an initial low threshold, with thresholds increasing as the program progresses.
What is a Warm-up Cycle?
A warm-up cycle is defined as driving the vehicle in a way that increases the engine coolant temperature by at least 40 degrees Fahrenheit after engine start and reaches a minimum of 140 degrees Fahrenheit for diesel engines (160 degrees for natural gas engines).
Why PDTCs are Ignored After Sufficient Operation
Once a vehicle’s OBD system completes its diagnostic tests after repair and sufficient operation, the PDTC should either trigger the MIL again if the fault persists or clear itself if the issue has been resolved. The Clean Truck Check program is phasing in operational limits to minimize disruption for vehicle owners adapting to the new requirements. These owners may be encountering challenges in getting all Monitors to run to completion and achieve readiness for testing. The minimum operation thresholds will be gradually increased as vehicle owners become more familiar with the testing requirements and integrate the scheduling and execution of these tests into their fleet operations.
Note: This information is intended for guidance and does not supersede the official Clean Truck Check Regulations.
