If you drive a diesel truck, SUV, or van in California, the smog check rules work a little differently than they do for gasoline vehicles. The test itself looks different, the schedule has some unique requirements, and the weight of your vehicle determines which program you fall under entirely.
Here is a clear breakdown of what diesel owners in Monterey Park and the surrounding San Gabriel Valley need to know.
Do Diesel Vehicles Need a Smog Check in California?
Yes, most diesel vehicles do. Whether your vehicle needs one depends on two things: its model year and its weight.
Light-duty diesel vehicles (under 14,001 lbs GVWR) Diesel cars, trucks, SUVs, and vans that are 1998 model year or newer and weigh 14,000 lbs or under follow the standard BAR Smog Check program. That means a smog test every two years for registration renewal, and whenever the vehicle changes ownership.
Heavy-duty diesel vehicles (over 14,000 lbs GVWR) Vehicles above 14,000 lbs are regulated under a separate program called Clean Truck Check (CTC), administered by the California Air Resources Board (CARB). This program operates independently from the standard BAR smog check and has its own compliance fees and testing schedule.
Exempt diesel vehicles Diesel vehicles from 1997 and older, or with a gross vehicle weight of more than 14,000 lbs, are exempt from the standard biennial BAR smog inspection — though heavy-duty vehicles over 14,000 lbs fall under the CTC program instead.
If you are unsure which category your vehicle falls into, check the GVWR on the sticker inside your driver’s side door jamb. That number tells you everything.
How Is a Diesel Smog Test Different?
The test for a diesel vehicle is not the same as a standard gasoline smog check. Diesel smog checks do not use tailpipe sniffer tests or dynamometers. Instead, the inspection focuses on diagnostics, emissions equipment, and visible smoke.
Here is what actually happens during a diesel smog inspection:
OBD diagnostic scan The technician connects a scan tool to your vehicle’s OBD port and reads the data from your onboard computer. This checks whether all emissions monitors have completed their self-tests. As of October 1, 2025, California requires that all applicable readiness monitors be complete for a vehicle to pass a smog check inspection. If your battery was recently disconnected or codes were cleared, the vehicle will report “Not Ready” and automatically fail.
Visual emissions equipment check The technician verifies that all factory emissions components are present and unmodified. Missing or altered equipment results in an immediate failure.
Visible smoke check The engine is briefly revved to check for lingering smoke. If visible smoke remains for more than three seconds, the vehicle fails.
This is why diesel vehicles that have been modified, had recent battery disconnections, or have cleared diagnostic codes may struggle to pass even if the engine itself is running fine. Your monitors need time to reset after a battery disconnect or code clear — typically a full drive cycle or more.
What If My Diesel Has Recently Had Work Done?
This is one of the most common reasons diesel vehicles fail smog in California. If your mechanic cleared a code or your battery was disconnected during a repair, your OBD readiness monitors reset to zero. The vehicle needs to be driven through a complete drive cycle before those monitors will show as complete.
If you bring the vehicle in immediately after repairs or a battery reset, there is a good chance the monitors will not be set and you will fail automatically.
The fix: drive the vehicle normally for several days covering a mix of highway and city miles before coming in for your test. This gives the onboard system time to run through its self-checks.
For diesel vehicles specifically, the monitors that need to complete include the catalyst monitor, EGR system monitor, and particulate filter monitor if your vehicle has a DPF.
Do Diesel Trucks Need a STAR Station?
Not necessarily, but it depends on what your DMV renewal notice says. Some diesel vehicles — particularly older ones or those with a history of failures — are directed to a STAR certified station for testing.
AB Smog Check in Monterey Park is a STAR certified station, so we can handle both standard smog tests and STAR-directed inspections for diesel vehicles. If your notice says STAR, you are in the right place.
How Much Does a Diesel Smog Check Cost in Monterey Park?
Pricing varies by station and vehicle. At AB Smog Check, diesel vehicles are priced separately from gasoline vehicles. Call us at 626-280-2678 for current pricing. We also offer $10 off with no coupon needed.
The state mandates an $8.25 certificate fee on top of the inspection cost, which is consistent across all licensed stations in California.
Tips to Help Your Diesel Pass the First Time
A little preparation goes a long way. Here is what to do before bringing your diesel in for a smog check:
Do not disconnect the battery or clear codes before your test. If your check engine light comes on before your scheduled smog check, get the underlying issue properly repaired rather than clearing the code and hoping it stays off. If codes were cleared, the vehicle will report “Not Ready” and automatically fail.
Drive the vehicle for several days before the test. This gives the OBD monitors time to run their self-checks and report as complete. Cold starts, highway driving, and in-town driving all contribute to different monitors completing.
Check for visible smoke before you come in. Start your vehicle and watch the exhaust. A brief puff at startup is normal for some diesels, but sustained smoke or heavy black or blue smoke is a problem worth addressing before your test.
Make sure your emissions equipment is intact. Do not remove or bypass your EGR, DPF, or catalytic converter. These are checked visually during the inspection and their absence is an automatic failure.
Do not fill the tank immediately before the test. A full tank can affect EVAP monitor readings. Drive with a normal fuel level.
What If My Diesel Fails?
If your diesel vehicle does not pass, the technician will provide a Vehicle Inspection Report (VIR) detailing exactly what failed and why. Common failure reasons for diesel vehicles include failed readiness monitors, visible smoke, missing or modified emissions equipment, and active fault codes.
Once repairs are made, you can return for a retest. At AB Smog Check, if we perform the repairs, the retest is free.
If the cost of repairs is significant and your household income qualifies, California’s Consumer Assistance Program (CAP) may be able to help cover up to $1,200 in smog-related repair costs.
Bring Your Diesel to AB Smog Check in Monterey Park
We test diesel vehicles at 501 E Garvey Ave, Monterey Park, CA 91755. No appointment needed. We are open Monday to Friday 9am to 5pm and Saturday 9am to 3pm.
As a STAR certified station, we handle standard diesel inspections and STAR-directed tests. Call 626-280-2678 if you have questions about your specific vehicle before coming in.
Frequently Asked Questions
Do all diesel vehicles in California need a smog check? Most 1998 and newer diesel vehicles under 14,001 lbs GVWR need a biennial smog check. Diesel vehicles from 1997 and older are exempt from the standard BAR smog check. Heavy-duty diesel vehicles over 14,000 lbs fall under the separate Clean Truck Check program administered by CARB.
Why did my diesel fail for “not ready” monitors? This means your OBD readiness monitors have not completed their self-tests, usually because the battery was recently disconnected or codes were recently cleared. Drive the vehicle normally for several days and return for a retest.
Can any smog station test my diesel vehicle? Most licensed BAR smog stations can test light-duty diesel vehicles. If your DMV notice specifically says STAR station, you need to go to a STAR certified station. AB Smog Check in Monterey Park is STAR certified.
Is the diesel smog check the same price as for a gasoline car? Not always. Diesel inspections are often priced differently from gasoline vehicle inspections. Call ahead to confirm pricing for your specific vehicle.
What is the Clean Truck Check program? The Clean Truck Check (CTC) is a separate CARB-administered emissions compliance program for heavy-duty diesel vehicles over 14,000 lbs GVWR. It operates independently from the standard BAR smog check program and has its own testing schedule and fees. More information is available at ww2.arb.ca.gov.
