What a smoke opacity test measures
A smoke opacity test measures how much smoke a diesel engine puts out when it is revved hard from idle. California uses it, along with a visual inspection, as the Clean Truck Check test for 2012 and older diesel engines without OBD.
Here is how the smoke opacity test is done. With the engine at normal operating temperature, our tester fits an opacity meter to the exhaust. The engine is then taken through a series of snap accelerations: quick full-throttle revs from idle, which is why some drivers call it a snap idle test. The meter reads the smoke from each snap, and the readings are combined into one result.
California smoke opacity limits are set by CARB and depend on the engine's model year, so two trucks with the same reading can get different results. We do not guess. After the test we tell you the reading, whether the truck passed and, if it did not, what usually causes that much smoke.
Want to improve your odds before test day? Read how to pass the CARB smoke test.
The visual inspection: labels, parts and tampering
Every diesel smoke test comes with a CARB visual inspection, and a truck has to pass both. A clean smoke reading does not save a truck that fails the visual check.
During the emissions visual inspection, our tester looks at:
- The emission control label (ECL) on the engine
- The emissions-control parts the engine should have
- Signs that parts are missing, disconnected, modified or tampered with
A tamper inspection on a diesel truck in California is strict. Deleted or tampered emissions equipment fails, and running it is illegal. We will not help with deletes. If a part is missing, the fix is to put the right part back and retest.
Older alternative-fuel trucks are a different case. A 2017 or older alternative-fuel engine without OBD does not take an opacity test at all. It gets the visual inspection only. Newer alternative-fuel engines use the CARB OBD test.
Smoke test for older diesel trucks, dump trucks and motorhomes
If your diesel engine is 2012 or older and has no OBD, the smoke opacity test plus visual inspection is your Clean Truck Check test. That is the main form of non-OBD truck testing in California.
A lot of older equipment still works hard in Southern California and falls in this group:
- Dump trucks and water trucks on construction jobs
- Agricultural and farm trucks
- Older tractors with engines such as the Detroit Series 60, Cummins N14 or Caterpillar C15
- Refuse, tow and utility trucks from before 2013
- Diesel motorhomes over 14,000 lb GVWR, where they fall under the program
Engine make does not change the test. A Detroit Series 60 smoke test and a Cummins N14 opacity test follow the same steps. What changes the result is condition: injectors, turbo, air intake, exhaust leaks and any particulate filter.
Not sure if your vehicle is covered? Check CARB's rules for your vehicle, especially for motorhomes and specialty equipment, or call us with the engine details.
Mobile smoke opacity test near you
We bring the opacity meter to the truck. Diesel Dynamix does mobile smoke opacity testing at yards, farms, jobsites, terminals and homes across Southern California, from Los Angeles and Orange County to Riverside, San Bernardino, San Diego and El Centro.
Testing on site matters more for older trucks. A cold drive to a test site does not help a smoke reading, and an older truck that breaks down on the way costs you a day. At your yard, the truck warms up, gets tested and goes back to work.
Smoke test cost depends on how many trucks we test, the test type and the location, with per-truck pricing for fleets. Mixed fleets can book smoke tests and OBD tests in the same visit.
Call or text (858) 342-8328, use WhatsApp, or book online.
