How the CARB Smoke Opacity Test Works (And Who Needs It)

What the Smoke Opacity Test Measures (SAE J1667)

The smoke opacity test is a snap-acceleration inspection that measures how much light is blocked by diesel exhaust during short throttle “snaps.” A credentialed tester uses an SAE J1667-compatible smoke meter to record opacity as a percentage.

• Method: SAE J1667 snap-acceleration performed in neutral with a certified smoke meter.

• Output: % opacity — lower numbers mean cleaner exhaust.

• Equipment: J1667-compatible meter; for non-OBD vehicles a visual inspection of emissions controls is also required.

For non-OBD vehicles, Clean Truck Check requires both the smoke opacity test and a visual inspection of emissions control equipment.

Who Needs a Smoke Opacity Test Under Clean Truck Check

Testing type is based on whether your engine is OBD-equipped. OBD vehicles submit electronic OBD data; non-OBD vehicles complete smoke opacity + visual inspection.

• OBD-equipped: 2013+ diesel engines and 2018+ alternative fuel engines — OBD scan (no opacity test).

• Non-OBD: 2012 and older diesel engines and 2017 and older alternative fuel engines — smoke opacity test + visual inspection.

• Notice to Submit to Testing (NST): Any vehicle flagged as a potential high emitter may be required to submit a passing compliance test within 30 days.

How Often & How Results Are Submitted

Most vehicles must submit two compliance tests per year (you can submit up to 90 days before your deadline). OBD results transmit via a CARB-approved device; non-OBD smoke/visual results are submitted by a credentialed tester through the CTC-VIS system.