---
title: "Williams Transmissions and Express Smog — Clean Truck Check, Napa, CA"
description: "Williams Transmissions and Express Smog in Napa, CA is on CARB's Clean Truck Check tester list. Offers mobile testing, OBD testing, motorhome testing. Phone…"
url: "https://ctcfinder.com/tester/williams-transmissions-and-express-smog-napa/"
updated: "2026-11-01"
publisher: "CTC Finder (independent directory, not affiliated with CARB)"
---

# Williams Transmissions and Express Smog

Clean Truck Check tester in Napa, CA (Napa County).

> CTC Finder is an independent directory, not affiliated with CARB. Tester data: CARB Clean Truck Check public tester list, last checked November 1, 2026. Confirm services, hours and prices with the tester.

| Field | Value |
|---|---|
| Location | 238 Soscol Ave, Napa, CA 94559 |
| County | Napa County |
| Phone | (707) 310-1999 |
| Mobile testing | Yes |
| OBD testing | Yes |
| Smoke opacity (non-OBD) | No |
| Motorhomes | Yes |
| Credentialed testers | 1 |
| Last verified | 2026-11-01 |

## Which trucks Williams Transmissions and Express Smog can test

| Your vehicle | Test required | This tester |
|---|---|---|
| Diesel, model year 2013 or newer | OBD scan | Yes |
| Alternative fuel (natural gas, propane), 2018 or newer | OBD scan | Yes |
| Diesel, model year 2012 or older (no OBD) | Smoke opacity test (SAE J1667) plus visual inspection | Not listed |
| California-registered motorhome | Same test as above by engine year, once a year | Yes |

For a 2012 or older diesel, the nearest listed tester with smoke opacity testing is Aaron Ramo in Napa, about 1 mile away. [See listing](https://ctcfinder.com/tester/aaron-ramo-napa/).

## Clean Truck Check testers in Napa County

Williams Transmissions and Express Smog is one of 11 testing businesses in Napa County on CARB's public tester list. 11 of them offer mobile testing and 6 do smoke opacity tests. In Napa itself there are 11 listed businesses, 11 of them mobile.

## When your truck is due

Most trucks test twice a year, on a schedule tied to the DMV registration expiration date. CARB says OBD trucks move to four tests a year beginning October 2027. California-registered (non-commercial) motorhomes and agricultural vehicles test once a year. See the [deadline guide](https://ctcfinder.com/guide/deadlines-2026-2027/). You can test up to 90 days before a deadline. Free email reminders: https://ctcfinder.com/reminders/

## Frequently asked questions

### Does Williams Transmissions and Express Smog offer mobile Clean Truck Check testing?

Yes. Williams Transmissions and Express Smog says on CARB's tester list that it offers mobile testing, so it can come to your yard or job site. Ask about a trip fee when you call.

### Can Williams Transmissions and Express Smog test a 2012 or older diesel truck?

Williams Transmissions and Express Smog does not list smoke opacity testing, which older non-OBD diesel trucks need. The nearest listed tester that does is [Aaron Ramo](https://ctcfinder.com/tester/aaron-ramo-napa/) (about 1 mile). Trucks from 2013 on get an OBD scan instead.

### How much does Williams Transmissions and Express Smog charge for a Clean Truck Check?

Williams Transmissions and Express Smog has not published prices with us; call (707) 310-1999 for a quote. Published California prices are usually $50–$135 per vehicle for an OBD test and $190–$200 for a smoke opacity test, plus CARB's annual fee ($32.13 in 2026). See the [cost guide](https://ctcfinder.com/guide/cost/).

### Is Williams Transmissions and Express Smog a CARB-credentialed tester?

Williams Transmissions and Express Smog appears on CARB's public Clean Truck Check tester list, last checked November 1, 2026. Only credentialed testers can submit results to CARB; after the test, confirm the passing result shows for your vehicle in CTC-VIS.

## Other testers nearby

- **[Aaron Ramo](https://ctcfinder.com/tester/aaron-ramo-napa/)** — Napa, CA 94559 — (707) 225-7982 — 1 mi. Services: mobile testing, OBD testing, smoke opacity (non-OBD) testing, motorhome testing.
- **[Jones Family Mobile Diesel Repair](https://ctcfinder.com/tester/jones-family-mobile-diesel-repair-napa/)** — 933 Water St, Napa, CA 94559 — (410) 301-8518 — 1 mi. Services: mobile testing, OBD testing, smoke opacity (non-OBD) testing, motorhome testing.
- **[Mobile Testing](https://ctcfinder.com/tester/mobile-testing-napa/)** — Napa, CA 94559 — (916) 538-7777 — 1 mi. Services: mobile testing, OBD testing, smoke opacity (non-OBD) testing, motorhome testing.
- **[Precision Automotive Services](https://ctcfinder.com/tester/precision-automotive-services-napa/)** — 198 Soscol Ave, Napa, CA 94559 — (707) 912-9318 — 1 mi. Services: mobile testing, OBD testing, motorhome testing.
- **[Quick Smog](https://ctcfinder.com/tester/quick-smog-napa/)** — 662 Soscol Ave, Napa, CA 94559 — (707) 927-3956 — 1 mi. Services: mobile testing, OBD testing, motorhome testing.
- **[Steve DiPiero](https://ctcfinder.com/tester/steve-dipiero-napa/)** — Napa, CA 94559 — (707) 996-3685 — 1 mi. Services: mobile testing, OBD testing, smoke opacity (non-OBD) testing.