People often use "smog check" as a catch-all for any emissions test — but in California, heavy-duty diesel trucks and RVs don't get a traditional Smog Check. They fall under a separate CARB program called Clean Truck Check. Here's the difference, and how to know which one your vehicle needs.
The Short Answer
- Smog Check: The California Bureau of Automotive Repair (BAR) program for most gasoline cars and lighter vehicles, done at licensed Smog Check stations.
- Clean Truck Check: The California Air Resources Board (CARB) program (officially HD I/M) for heavy-duty vehicles over 14,000 lbs GVWR — including diesel trucks, buses, and motorhomes.
If you operate a heavy-duty diesel, the Smog Check program does not apply to you — Clean Truck Check does. New to the program? Start with what Clean Truck Check is.
Side-by-Side Comparison
| Smog Check | Clean Truck Check | |
|---|---|---|
| Run by | BAR (Bureau of Automotive Repair) | CARB (Air Resources Board) |
| Applies to | Most gas cars & light vehicles | Diesel vehicles over 14,000 lbs GVWR |
| Where | Licensed Smog Check stations | Mobile or self-reported to CARB |
| How often | Every 2 years (biennial) | Up to twice a year for trucks |
| Records to | DMV via BAR | CTC-VIS database |
Which Test Does My Diesel Need?
Within Clean Truck Check, the exact test depends on your engine's model year — not the chassis year:
OBD Test
2013 & Newer Engines
A technician connects to the vehicle's on-board diagnostics (OBD) port, downloads emissions data and fault codes, and uploads the results to CARB.
Opacity Test
Pre-2013 Engines
A tailpipe smoke (opacity) test measures particulate output, paired with a visual inspection of emissions equipment like the DPF.
For the full list of who must comply and how often, see our 2026 CARB requirements guide.
Don't Get Caught by the Wrong Program
Assuming your diesel just needs a "smog check" is a common — and costly — mistake. Missing your Clean Truck Check deadline triggers the same registration holds and fines regardless of confusion. We handle both test types at your location — schedule a check or view pricing.
Still Not Sure?
If you're unsure whether your vehicle needs a Smog Check or a Clean Truck Check, our team can confirm it in a quick call and identify the correct test on-site. See our mobile services or reach out through our contact page.