The short version
| Engine | Test |
|---|---|
| Diesel, 2013 and newer | OBD test |
| Diesel, 2012 and older | Smoke opacity test plus a visual inspection of the emissions control equipment |
| Alternative fuel, 2018 and newer | OBD test |
| Alternative fuel, 2017 and older | Visual inspection only (no opacity test) |
Go by the engine's model year, not the truck's. CARB calls the older groups “non-OBD” vehicles.
The OBD test
For vehicles with an acceptable onboard diagnostic (OBD) system, the credentialed tester connects a CARB-approved OBD download device to the truck, collects the data, and submits it to CARB. CARB publishes a list of its approved OBD test devices.
The opacity test and visual inspection
Older diesel engines without OBD get two checks. The first is a smoke opacity test, which uses a snap-acceleration procedure (CARB points to SAE J1667 as a reference). The second is a visual inspection of the vehicle's emissions control equipment, which CARB calls the Vehicle Emissions Control Equipment Inspection. Older alternative fuel engines only get the visual inspection.
Who can perform the test
Only a CARB-credentialed Clean Truck Check tester can perform either test and submit it. To become one, a person completes CARB's Heavy-Duty Inspection and Maintenance tester course, passes the final exam, and is issued a tester ID. Testers must use CARB-approved OBD devices or opacity testers. Owners who need a tester can use CARB's list of credentialed testers available for hire.
What this means for pricing your work
If you test for a living, the two jobs aren't the same. An OBD test is a connect-and-submit job, while an opacity test adds a smoke test and a hands-on inspection of the equipment, so you may want to price them separately.
In SmogDesk, Clean Truck OBDII and Clean Truck Opacity are separate services, each with its own price and cert fee, so every invoice shows the right test at the right price. For the full walkthrough, see how to invoice a Clean Truck Check test.
Rules change. For current requirements, see CARB's credentialed tester overview and emissions compliance testing requirements. This guide reflects CARB's published information as of October 2026.