California Air Resources Board

Clean Truck Check OBD Testing Background

Vehicles subject to OBD testing

• Heavy-duty (Gross Vehicle Weight Rating ≥ 14,000 lbs.) diesel vehicles and diesel hybrids with 2013 and newer model year engines.
• Alternative fuel (e.g., natural gas) and alternative fuel hybrid heavy-duty vehicles with 2018 and newer model year engines.

OBD system purpose

• Monitors the effectiveness of the Emission Control Systems (ECS)
• Identifies ECS in need of repair
• Notifies driver when problems occur
• Helps technicians diagnose and repair
• Encourages the design of robust and durable ECS

Core elements of the Clean Truck Check OBD test: 

• Malfunction Indicator Light (MIL) status and associated Diagnostic Trouble Codes (DTCs) 
• Monitor Readiness
• Operation Since codes Cleared (OSC) 
• Permanent Diagnostic Trouble Codes (PDTCs) 

MIL status

The OBD scan device communicates with the vehicle’s on-board computer about faults known as DTCs. If a DTC is currently causing the MIL to be commanded ON, there is an emissions related issue with the vehicle. Thus, the vehicle will fail the test.

Monitor readiness

An OBD system stores information that indicates if it is “ready” for a compliance test. A vehicle is not ready (and cannot pass a compliance test) if it has not had enough operation since the OBD memory was last cleared (also known as a “code clear”), and thus has not had sufficient time to complete a full diagnostic assessment to determine if an emissions related issue exists.

A code clear can occur if a fault has recently been repaired, or if a vehicle recently has had a dead, disconnected, or replaced battery. It does not necessarily mean that anything is wrong with the vehicle - it simply means that the vehicle hasn't had a chance to run all its self-diagnostics to confirm that it is ready for compliance testing. The vehicle will need to be driven more before it can be tested.

Vehicle OBD systems have many components and operations that are monitored. These elements are categorized into Monitor groups. The chart below is a somewhat generic consolidated list of Monitors. The actual Monitor components within a Monitor group may vary between vehicle manufacturer, engine manufacturer, fuel type, model year, and communication protocol. Below are examples of readiness Monitor groups.

Based on tests performed to date in Clean Truck Check, most vehicles have all Monitors ready at the time compliance tests are performed and submitted to CARB. However, vehicles that have had codes cleared recently will have multiple Monitors not ready (incomplete).

Operation Since codes Cleared (OSC)

OBD systems store information regarding vehicle operation since the diagnostic trouble codes were last cleared. Most vehicles have OSC for:

• Warm-Up Cycles (WUC) since cleared
• Distance travelled, in km, since cleared
• Engine run time, in minutes, since cleared

What is a warm-up cycle?

A warm-up cycle means driving a vehicle so that the engine coolant temperature rises by at least 40 degrees Fahrenheit after the engine is started and reaches at least 140 degrees Fahrenheit for diesel engines (160 degrees for natural gas engines).

Vehicle operation needed for Monitors to be ready

Heavy-duty diesel vehicles require operation to complete readiness. For example, based on OBD data already submitted to the Clean Truck Check, dozens of Warm-up Cycles, thousands of miles, and hundreds of hours of engine run time are required for all Monitors to be ready. That is why it is essential to be proactive and repair emissions-related problems ASAP and to avoid clearing codes if at all possible.

Permanent Diagnostic Trouble Codes

What are Permanent Diagnostic Trouble Codes?

Permanent Diagnostic Trouble Codes (PDTCs) are very similar to regular Diagnostic Trouble Codes (DTCs). However, unlike regular DTCs, they cannot be reset by disconnecting the vehicle’s battery or cleared using an OBD scan device. The only way to clear a PDTC is to fix the underlying problem with the vehicle that originally caused the PDTC to set, and then allow the vehicle sufficient drive time to re-run the Monitor that identified the problem in the first place. When the Monitor runs without identifying a problem, the PDTC will clear itself.

Why are PDTCs being included in the Clean Truck Check Program?

Unplugging the vehicle’s battery or using a scan device to “clear codes” are techniques sometimes used to clear OBD information from a vehicle that has an illuminated MIL in an attempt to hide the fact that the vehicle is malfunctioning. Without the consideration of PDTCs, some of these vehicles can incorrectly pass a Clean Truck Check compliance test, which can have a dramatic impact on air quality and decrease the program’s effectiveness. Although the use of readiness indicators reduces the chances of passing a Clean Truck Check compliance test with an active DTC, PDTCs can further ensure emission control systems are working correctly.

How are PDTCs going to be used as part of a Clean Truck Check compliance test?

Upon implementation, vehicles that have a PDTC stored in the OBD system will fail the Clean Truck Check OBD test regardless of whether the malfunction indicator light is illuminated. If a PDTC is stored, it indicates that the OBD system has not yet successfully verified that a previously detected emissions-related malfunction is no longer active.

Are there circumstances under which a PDTC will not cause a vehicle to fail a Clean Truck Check compliance test?

Yes. PDTCs will be ignored if the vehicle meets or exceed Operation Since Clear criteria specifically for PDTCs (as opposed to general OSC criteria).

Why will PDTCs be ignored when the vehicle has completed a reasonable amount of operation since the codes were cleared?

Once a vehicle’s OBD system has completed its self-diagnostic tests, the PDTC should either set the MIL on if the fault still exists or clear itself if the fault has been repaired. As the Clean Truck Check program ramps up, operation limits are being established to minimize impacts on business operations to vehicle owners who are still getting used to the program and trying to comply, but may be having trouble getting specific Monitors to run to completion and ready for testing. The minimum operation thresholds will continue to ramp up over time as vehicle owners get used to these testing requirements and the scheduling and performance of these tests gets incorporated into fleet operation plans.

Note: The information above is not all-inclusive and does not replace or supersede any of the CTC Regulations.