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

The smoke opacity test checks how clean an older diesel engine’s exhaust is. Here’s how it works and who needs it.

Who needs it

• Diesel engines 2012 and older over 14,000 lbs GVWR: smoke opacity test plus a visual inspection
• Diesel 2013 and newer: no opacity test. They get an OBDII scan instead.
• Alternative-fuel engines 2017 and older: visual inspection only

How the test works

1. We check that the emissions parts are there and working.
2. We put a certified smoke meter on the exhaust.
3. The engine is revved quickly a few times. This is called a “snap acceleration” test.
4. The meter measures how much light the smoke blocks. Lower is cleaner.
5. We submit the results to CARB.

How often

Most vehicles test twice a year. California motorhomes and farm vehicles test once a year. You can submit up to 90 days before your deadline.

Got a Notice to Submit?

If roadside monitoring flags your truck, CARB sends a Notice to Submit. You’ll have 30 days to pass a test. Text us right away.