Carb Requirements

On-Board Diagnostic (OBD) systems are self-diagnostic systems incorporated into the computers of vehicles that monitor virtually every component that can affect vehicle emissions. If a problem with an emissions-related component is detected, the OBD system illuminates a warning lamp on the vehicle instrument panel to alert the driver and also stores information about the detected malfunctions to aid any needed repairs.

For these OBD systems to accurately monitor and detect emissions related problems, the vehicle must be operated a sufficient amount of time for the system to complete a full diagnostic monitoring cycle of each component it is monitoring. Once this occurs, the OBD system is in a ready state to accurately determine whether a vehicle has an emissions related performance issue or not. If a vehicle’s OBD system has been reset or recently cleared, also referred to as “code clearing,” it will take time and operation to bring the vehicle’s OBD system back into a fully ready state. Often a reset of the OBD system happens when the battery is disconnected or a scan tool is used to perform a code clearing.

The Clean Truck Check OBD readiness testing requirement is an adjustment for some vehicle owners. Understanding this, the program will ease into this OBD readiness requirement ramp up as the program matures. Until this time, the program will establish minimum operation thresholds that will increase in stringency until these full readiness requirements are achieved.

Current OBD testing Readiness Criteria

The current minimum operation threshold for a Clean Truck Check test submission is for the vehicle to have at least five (5) Warm-Up Cycles (WUCs) since “codes cleared”.

An OBD test submitted to the Clean Truck Check that does not meet this minimum operation threshold will get a Not Ready (not passing) test result and will not meet the emissions compliance testing requirements.

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

Getting ready for the Clean Truck Check OBD test

Things to do to have your best chance at passing the Clean Truck Check OBD test:

• If your Malfunction Indicator Light is on, get your vehicle repaired as soon as possible.

• Don’t wait. Do the OBD test early in the compliance window so there will be time to fix any issues and re-test your vehicle, if necessary, by the compliance deadline.

• Avoid clearing codes. OBD systems require a lot of vehicle operation to be able to complete the emission control system Monitor checks. A vehicle that has not been operated enough will fail the compliance test. Code clearing can occur when:

  • Disconnecting the battery

  • Using an improperly installed kill switch that disconnects power to the OBD system (it is OK if the kill switch just cuts power to accessories/lights/etc.)

  • Using an OBD scan tool to clear codes

• Please ensure the test results appear in your CTC-VIS account when getting your vehicle(s) tested. If the test results do not show up within five minutes after completing the test, you may need to:

  • re-test your vehicle.

  • unplug other devices connected to the diagnostic port.

  • verify the communication between the vehicle and the OBD test device.

  • ensure the internet connection is stable.

• If the results still do not appear after multiple attempts during your emissions compliance testing, please email us at hdim@arb.ca.gov.​​​​​​​

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.

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

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).

On-Board Diagnostic (OBD) systems are self-diagnostic systems incorporated into the computers of vehicles that monitor virtually every component that can affect vehicle emissions. If a problem with an emissions-related component is detected, the OBD system illuminates a warning lamp on the vehicle instrument panel to alert the driver and also stores information about the detected malfunctions to aid any needed repairs.

For these OBD systems to accurately monitor and detect emissions related problems, the vehicle must be operated a sufficient amount of time for the system to complete a full diagnostic monitoring cycle of each component it is monitoring. Once this occurs, the OBD system is in a ready state to accurately determine whether a vehicle has an emissions related performance issue or not. If a vehicle’s OBD system has been reset or recently cleared, also referred to as “code clearing,” it will take time and operation to bring the vehicle’s OBD system back into a fully ready state. Often a reset of the OBD system happens when the battery is disconnected or a scan tool is used to perform a code clearing.

The Clean Truck Check OBD readiness testing requirement is an adjustment for some vehicle owners. Understanding this, the program will ease into this OBD readiness requirement ramp up as the program matures. Until this time, the program will establish minimum operation thresholds that will increase in stringency until these full readiness requirements are achieved.

Current OBD testing Readiness Criteria

The current minimum operation threshold for a Clean Truck Check test submission is for the vehicle to have at least five (5) Warm-Up Cycles (WUCs) since “codes cleared”.

An OBD test submitted to the Clean Truck Check that does not meet this minimum operation threshold will get a Not Ready (not passing) test result and will not meet the emissions compliance testing requirements.

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

Getting ready for the Clean Truck Check OBD test

Things to do to have your best chance at passing the Clean Truck Check OBD test:

• If your Malfunction Indicator Light is on, get your vehicle repaired as soon as possible.

• Don’t wait. Do the OBD test early in the compliance window so there will be time to fix any issues and re-test your vehicle, if necessary, by the compliance deadline.

• Avoid clearing codes. OBD systems require a lot of vehicle operation to be able to complete the emission control system Monitor checks. A vehicle that has not been operated enough will fail the compliance test. Code clearing can occur when:

  • Disconnecting the battery

  • Using an improperly installed kill switch that disconnects power to the OBD system (it is OK if the kill switch just cuts power to accessories/lights/etc.)

  • Using an OBD scan tool to clear codes

• Please ensure the test results appear in your CTC-VIS account when getting your vehicle(s) tested. If the test results do not show up within five minutes after completing the test, you may need to:

  • re-test your vehicle.

  • unplug other devices connected to the diagnostic port.

  • verify the communication between the vehicle and the OBD test device.

  • ensure the internet connection is stable.

• If the results still do not appear after multiple attempts during your emissions compliance testing, please email us at hdim@arb.ca.gov.​​​​​​​

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.

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

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).

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

Almost all vehicles in Clean Truck Check track WUCs. If a vehicle does not track WUCs, a CARB-certified OBD scan device will next look for distance traveled, and if that is not tracked then finally to engine run time. Readiness criteria will therefore be applied to only one OSC variable provided by an individual vehicle, and vehicles that do not have at least a minimum level of OSC for that variable will have a test result of Not Ready. As they are phased in, the criteria thresholds for these variables will become more stringent over time.

Vehicle operation needed for Monitors to be ready

Heavy-duty diesel vehicles require regular 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.

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Vehicle Emissions Control Equipment Inspection

Applies to heavy-duty vehicles without On-Board Diagnostic (OBD) systems

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General Information

The Vehicle Emissions Control Equipment Inspection is a requirement under **Clean Truck Check** regulations.

Vehicle Applicability

The inspection applies to **non-OBD heavy-duty vehicles** receiving a **Notice to Submit to Testing** starting in **January 2023**. The compliance testing requirement begins **October 1, 2024**.

Non-OBD vehicles include:

• 2012 or older diesel engines/diesel hybrid systems
• 2017 or older alternative fuel engines/hybrid systems
• On-road vehicles with **off-road engines** (approved via CARB-issued waiver)

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Completing the Inspection Form

Vehicle & Tester Information

• Fill in all required **vehicle, engine, and tester details**.
• Forms should be **submitted electronically** whenever possible.
• If handwritten, ensure **clear and legible writing**.

Vehicle Emissions Control Equipment Checklist

Each component must be inspected and recorded as one of the following:

• Pass - Component is functioning correctly.
• Needs Repair - Component is defective/missing.
• Not Applicable - Component does not apply to the vehicle.

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Visual Inspection Criteria

Components to Inspect:

Component	Description
Exhaust Gas Recirculation (EGR)	Includes EGR valve, EGR cooler, controls
Selective Catalytic Reduction System (SCR)	Includes SCR catalyst, Diesel Exhaust Fluid (DEF) system, monitoring system
Three-Way/Oxidizing Catalyst (TWC/OC)	Includes catalysts, monitoring systems, warning lights
Sensors and Switches	Includes oxygen sensors, air-fuel sensors, coolant temp, NOx sensors
Engine Control Module/Powertrain Control Module (ECM/PCM)	Includes computers, modules, wiring, controls, actuators, connectors, warning lights
Diesel Particulate Filter (DPF)	Includes filter, regeneration system, monitoring system
Retrofit DPF:  Verified Diesel Emissions Control Strategy (VDECS)	Includes DPF criteria above (for vehicles originally equipped with a DPF) as well as inspection of intake and exhaust manifolds, air filter including flow indicator, crankcase controls, related hoses, connectors, brackets, and hardware; other emissions related components for a particular vehicle/engine as determined from the manufacturer's specifications, emission control label, certification data, or published vehicle parts manuals

❗ Vehicles with **missing, modified, defective, or disconnected** emission control components will fail compliance testing.

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Credentialed Clean Truck Check Tester Certification

The tester must **sign, print their name, and enter the date of inspection** to certify that the inspection was performed correctly.

❗ Submitting **false or incomplete** inspection reports may result in penalties.

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Submitting the Inspection Results

• Only **credentialed Clean Truck Check testers** may submit the form.
• CARB will send an **auto-reply confirmation** upon receipt.

For CARB Enforcement Testing:

• Email: hdvehicleenforcement@arb.ca.gov

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