FAQ: CARB Clean Truck Check Program Guide

1. What is the California Clean Truck Check (HD I/M) program?

The Clean Truck Check (Heavy-Duty Inspection and Maintenance, or HD I/M) program is a regulation implemented by the California Air Resources Board (CARB) to ensure that heavy-duty vehicles operating in California have properly functioning emissions control systems. Starting in January 2023, this program aims to identify and repair polluting heavy-duty vehicles to significantly reduce smog-forming and carcinogenic toxic air pollution. It applies to nearly all diesel and alternative fuel heavy-duty trucks and buses with a Gross Vehicle Weight Rating (GVWR) over 14,000 pounds, including commercial, privately-owned, government, hybrid vehicles, and California-registered motorhomes, even if registered outside California.

2. Why is the Clean Truck Check program necessary?

California, especially densely populated areas like the South Coast and San Joaquin Valley, exceeds federal ozone and PM2.5 standards, leading to poor air quality. Heavy-duty vehicles, despite being only 6% of California’s vehicles, contribute a quarter of the state’s on-road greenhouse gas emissions and over a third of its NOx emissions. Diesel particulate matter (PM) is a known carcinogen, and NOx is a precursor to ozone and other toxic air contaminants. The HD I/M program is crucial for meeting federal air quality mandates, improving public health in disproportionately impacted communities, and preventing serious health effects like asthma, cardiopulmonary, and respiratory diseases. The program is projected to reduce NOx emissions by 8.6 tons per day and PM2.5 emissions by 0.09 tons per day in the San Joaquin Valley by 2024, leading to an estimated 7,500 avoided premature deaths and $76 billion in health benefits statewide by 2037.

3. What are the key phases and requirements of the Clean Truck Check program?

The Clean Truck Check program is being phased in with several key requirements:

• Phase 1 (January 2023) — Roadside Emissions Monitoring (REMD): CARB began using roadside emissions monitoring devices to screen for high-emitting vehicles. If a vehicle is flagged, the owner receives a “Notice to Submit to Testing” (NST) and has 30 calendar days to submit a passing compliance test.
• Phase 2 (Mid-2023) — Owner Accounts and Fees: Vehicle owners were required to create accounts in CARB’s HD I/M database (CTC-VIS), verify their vehicle information, and pay an annual compliance fee (e.g., $31.18 for 2025). This also includes public agencies, freight contractors, and brokers verifying compliance for vehicles they contract with.
• Phase 3 (January 2025 onwards) — Periodic Compliance Testing: All compliance deadlines on or after January 1, 2025, require the submission of a passing emissions compliance test by a CARB credentialed tester. Testing can be done up to 90 days before the deadline. Deadlines for CA-registered vehicles are based on registration dates, and for out-of-state vehicles, on the last digit of the VIN.

Failure to comply can result in citations for non-compliance, DMV registration blocks, and additional penalties.

4. How are vehicles tested under the Clean Truck Check program?

The type of emissions compliance test depends on the vehicle’s engine model year and its diagnostic capabilities:

• OBD-equipped vehicles (2013 and newer diesel engines, 2018 and newer alternative fuel engines): These vehicles undergo a scan of their On-Board Diagnostics (OBD) data using a CARB-certified OBD test device. OBD systems are crucial for monitoring and reporting the performance of various vehicle subsystems, generating Diagnostic Trouble Codes (DTCs) when anomalies are detected.
• Non-OBD vehicles (2012 and older diesel engines): These vehicles require a smoke opacity test and a visual inspection of the vehicle’s emissions control equipment, known as the Vehicle Emissions Control Equipment Inspection. The smoke opacity test uses the SAE J1667 snap-acceleration smoke inspection procedure. Alternative fuel non-OBD vehicles (2017 and older) only need the visual inspection.

5. What is the SAE J1667 Snap-Acceleration Smoke Test?

The SAE J1667 Snap-Acceleration Smoke Test is a recommended practice for measuring exhaust smoke from heavy-duty diesel-powered vehicles. It’s a non-moving vehicle test that can be conducted roadside or in other facilities. The procedure involves rapidly depressing the throttle to full open, holding it until maximum governed speed, then releasing it, allowing the engine to return to idle. This cycle is repeated, and smoke is measured using a smokemeter based on the light extinction principle. The test is designed to assess smoke emissions from in-use vehicles, indicating engine and fuel system maintenance or tampering issues, and is intended to identify “gross emitters” rather than replicating federal engine certification cycles.

6. What are common diesel engine problems that can cause a vehicle to fail an emissions test?

Several issues can lead to a diesel vehicle failing an emissions test, often related to incomplete combustion and excessive smoke:

• Excessive Smoke from Exhaust — Different smoke colors indicate specific problems:
  • Black Smoke: Too much fuel, often due to faulty injectors, restricted airflow (clogged filters, intake buildup, clogged intercoolers), or turbocharger issues (low boost pressure).
  • White Smoke: Unburned fuel (especially during cold starts), low compression, malfunctioning injectors, or burning coolant (blown head gasket, cracked cylinder head, leaking EGR cooler).
  • Blue Smoke: Burning oil, usually from worn piston rings, damaged valve stem seals, leaking turbo seals, or a failing PCV system.
• Loss of Power or Poor Performance: Can be caused by clogged air/fuel filters, malfunctioning turbocharger, restricted fuel delivery, or faulty engine sensors. This can also trigger a check engine light, providing diagnostic codes.
• Fuel Contamination and Injector Problems: Water, dirt, microbial growth, or low-quality fuel can clog filters, damage injectors, and lead to rough idling, knocking, or reduced fuel efficiency.
• Electrical and Sensor Failures: Modern diesel engines rely on numerous sensors (MAF, MAP, fuel rail pressure, oxygen sensors) and electronic control systems (ECM, ECU). Corroded wiring, loose connections, or worn-out sensors can lead to erratic performance, misfires, poor fuel economy, or warning lights.

Addressing these issues through regular maintenance, filter replacement, injector service, and electrical system checks is crucial for passing emissions tests.

7. How are smokemeters and data processing used in the SAE J1667 test?

The SAE J1667 test requires a smoke measurement and data-processing system consisting of a smokemeter (full-flow end-of-line or sampling type), a data-processing unit, and a recording medium. The smokemeter uses the light extinction principle to measure smoke opacity (percentage of light blocked) or smoke density (light extinction coefficient). NorCal CARB Mobile uses a Wager 6500+ purchased in 2025.

The data-processing unit is crucial for several functions:

• Filtering: It uses a second-order Bessel low-pass digital filter algorithm to process raw smoke readings and achieve a required overall instrument response time of 0.500 seconds ± 0.015 seconds, filtering out high-frequency noise.
• Corrections: It applies Beer-Lambert corrections to convert measured smoke values to standard reporting units, accounting for effective optical path length (Lm, Ls). It also performs light source wavelength corrections if a red LED is used, to ensure comparability with standard green LED or incandescent light sources.
• Calculation and Reporting: It calculates the maximum 0.5-second average smoke values for each snap-acceleration cycle, and then the final test result is the average of these three corrected values, provided test validation criteria are met.

Proper calibration (zero and full scale checks) and maintenance of the smokemeter are essential for accurate results.

8. What are the benefits of mobile Clean Truck Check services for vehicle owners?

Mobile Clean Truck Check services offer significant advantages for diesel truck owners and fleet managers in California, particularly for compliance with the CARB Clean Truck Check (HD I/M) program. Services like NorCalCARBMobile.com provide:

• Convenience: Certified technicians travel directly to the vehicle’s location (job site, truck yard, office), eliminating the need for owners to drive to a shop.
• Reduced Downtime: Testing can be performed while the vehicle is parked or the operator is working, saving valuable time and minimizing disruption to operations.
• DMV Compliance: These services ensure trucks meet California’s emissions standards and help avoid DMV registration holds and late fees.
• CARB Certified: Mobile providers use CARB-certified equipment and have credentialed testers.
• Fast, Digital Reporting: Test results are digitally submitted directly to CARB’s CTC-VIS portal, streamlining the compliance process.
• Cost Savings: For large fleets, competitive pricing can offer substantial savings compared to traditional shop visits. Referral programs also exist, allowing customers to earn rewards for referring other trucks.