Diesel smog check in California, who needs it and what is tested

Do diesel vehicles need a smog check in California

Yes, most do. Diesel cars, pickups and vans need a smog check if they are model year 1998 or newer and weigh 14,000 pounds or less gross vehicle weight rating. The Bureau of Automotive Repair, or BAR, runs that program. Diesel smog checks began on January 1, 2010.

The quick version is below. If your diesel is older than 1998 you are exempt. If it is over 14,000 pounds you skip the smog check and join CARB's Clean Truck Check instead.

Hybrids that burn diesel are covered too. The rules also cover diesel used alone or in combination with propane, natural gas, methanol or ethanol.

What year diesel is exempt

Diesel vehicles from model year 1997 and older do not need a smog check. That is the diesel cutoff. It is not the same as the gas cutoff, which is 1975 and older. A 1990 gas car still needs a smog check, but a 1990 diesel does not.

Diesels over 14,000 pounds also skip the BAR program. Those vehicles fall under Clean Truck Check, which tests trucks twice a year. See the Clean Truck Check guide.

Why diesel gets no new car break

Gas cars have two breaks. They skip the renewal check for eight model years, and they skip the sale check for four. Diesel gets neither. BAR says there are no model year exceptions for diesel vehicles. The laws that cut diesel out are Health and Safety Code section 44011 for the renewal break and Vehicle Code section 4000.1 for the sale break.

The practical result is that a one year old diesel truck sold to a new owner needs a smog check, unless the transfer is exempt for another reason, such as a sale between close family. The same truck in gasoline would not. A new diesel also needs one at its first renewal, where a gas car in the same year would wait eight years.

A smog certificate is valid for 90 days, the same as for gas cars. Plan the test close to the sale date. See the guide on how the exemptions changed over time if you are curious why diesel is treated differently.

What the diesel smog check looks at

Diesel vehicles from 1998 on get the OBD focused inspection. A BAR approved device plugs into the diagnostic port and reads the vehicle's computer. There is no probe in the tailpipe. A station that tests a 1998 or newer diesel on the old tailpipe equipment can be cited.

• Check engine light. The light must come on with the key on and engine off, and it must go out when the engine runs. If it stays on or flashes, the vehicle fails. BAR says a vehicle will not pass a smog check with the check engine light on.
• Visual inspection. The inspector looks at emissions parts and for fuel leaks.
• Visible smoke. This test applies to gasoline and diesel vehicles. Visible smoke fails.
• Readiness monitors. These are the computer's self tests. Diesels have strict limits, shown below.
• Permanent codes. On 2010 and newer vehicles, a stored permanent code fails the vehicle even when the light is off. The exception is a vehicle that has done 15 warm up cycles and 200 miles since the codes were cleared.
• Software check. Illegally modified software fails. Software with a CARB Executive Order passes.

A typical inspection takes around 30 minutes.

Readiness monitor rules for diesel

Diesels follow tighter monitor rules than many gas cars. A diesel from 1998 to 2006 may not have any monitor incomplete. A 2007 or newer diesel may have the particulate filter monitor and the NMHC catalyst monitor incomplete. The NMHC allowance was added on July 5, 2023.

On October 1, 2025 a rule took effect that will require all readiness monitors to be complete. BAR says the old pass and fail limits stay in place while it collects data. Changes will be phased in by vehicle and announced well in advance. If you are shopping for a used diesel, ask the seller to show that the monitors are ready.

A battery change or a code clearing resets the monitors. BAR says to drive one to two weeks before you test. A diesel often needs a mix of cold starts, highway driving and idle time to set its monitors. Do not rush to the station right after work on the truck.

Why a diesel fails smog

BAR does not publish fail rates by fuel type, so there is no diesel only number. The reasons that apply to diesels are clear from the test.

• Check engine light on. Fix the fault first, then clear the code and drive.
• Monitors not ready. Across all fuels, not ready monitors made up about half of newer car failures in 2025.
• Emissions equipment removed or changed. This is tampering. A tampered vehicle fails the visual check and cannot get a repair cost waiver.
• Modified software. A tune that is not approved by CARB fails.
• Visible smoke. A smoking diesel fails. The Smog Check Referee grants a waiver on a smoke failure only to low and moderate income owners.
• Stored permanent codes. Clearing codes right before the test does not hide them.

Deleted diesels will not pass. If the particulate filter, the exhaust gas recirculation system or other parts are gone, the vehicle fails and tampering rules apply. The fix is to put approved parts back.

How many diesels are in California

DMV counted 1,264,257 diesel and diesel hybrid vehicles at the end of 2024. That is 4.1% of the 31,141,961 vehicles on the books. DMV marks 599,938 of them light duty and 664,319 heavy duty. The line between light and heavy in the DMV file is not the same as the 14,000 pound line, so treat these as a size guide.

Newer diesels dominate the registration file. The chart shows model years 2011 to 2025. Many of the older vehicles are in a single group that DMV calls 2010 and older. It includes some diesels from 1997 and before that are exempt.

How to pass a diesel smog check

1. Confirm the weight. Read the gross vehicle weight rating on the door sticker. Over 14,000 pounds means Clean Truck Check.
2. Check your DMV notice. If it says STAR, you must test at a STAR station. Use the station finder and ask whether the shop tests diesels.
3. Scan for codes. Have the vehicle scanned before the test. Fix codes and drive one to two weeks so monitors set.
4. Look for missing parts. Every emissions part must be on the vehicle.
5. Take the test. It takes about 30 minutes. Pay the $8.25 state certificate fee when the vehicle passes.
6. If it fails, read the vehicle inspection report. Check whether the emissions warranty covers the repair.

Federal warranty covers the catalytic converter and engine computer for 8 years or 80,000 miles. California certified vehicles carry longer coverage on some parts. Call the CARB helpline at (800) 242-4450 if you are not sure.

What the filter and catalyst monitors mean

Two monitors matter most on a newer diesel. The particulate filter monitor checks the filter that traps soot from the exhaust. The NMHC catalyst monitor checks the part that cleans up unburned hydrocarbons. These monitors take a long drive to finish, and they can drop out after the computer is reset. That is why a 2007 or newer diesel is allowed to have them incomplete under today's rule.

That allowance may not last. The all monitors rule on the books since October 2025 would remove it. BAR has said it will first collect data and then phase changes in by vehicle. If you own a diesel, keep up with BAR news and do not count on the allowance forever.

Pickups, vans and motorhomes with diesel engines

The weight line decides most borderline cases. A diesel half ton or three quarter ton pickup is normally in the BAR program. Some heavier trucks, such as large dually pickups, cargo vans and chassis cab trucks, can carry a rating above 14,000 pounds. Look at the sticker on the driver door jamb and find the line that says gross vehicle weight rating. That number, and not the number on a sales brochure, is the one that counts.

Diesel motorhomes work the same way. A smaller diesel motorhome at 14,000 pounds or less that is model year 1998 or newer needs a smog check. A California registered motorhome over 14,000 pounds is a Clean Truck Check vehicle and tests once a year. An out of state motorhome over that weight is not covered by Clean Truck Check.

Where diesel smog checks apply

A diesel follows the same area map as a gas car. In biennial areas it needs a check every other year. In rural change of ownership areas it needs one only when it is sold or registered for the first time. Your registration ZIP code decides your area. Look up your county on the county lists to see which one you are in.

The test itself is the same wherever you live. You can test at any licensed smog station that has the right equipment, unless your DMV notice says STAR. Not every shop likes diesels, so call first and ask. A STAR station is held to higher standards and can test any vehicle, including directed ones.

Money, warranty and help

BAR does not set prices and does not publish a separate diesel price. In 2025 the average inspection price for all vehicles was $67.20, and the state certificate fee of $8.25 is extra on a pass. A station must give you a written estimate that you approve before it starts work.

If your diesel fails, check the warranty first. California certified vehicles carry free repairs for smog failures in the first 3 years or 50,000 miles, and for high priced parts for 7 years or 70,000 miles. Federal law covers the catalytic converter and engine computer for 8 years or 80,000 miles. A dealer may fix the problem at no cost.

If you have a low income and a failed smog check, the Consumer Assistance Program may pay up to $1,450 toward repairs at a STAR test and repair station on a 1996 or newer vehicle. It may also pay $1,350 to $2,000 to retire a vehicle that is in the smog check program. Read the vehicle retirement guide for how that works. Smog repair costs can also qualify you for a repair cost waiver after $650 in repairs, but not if the vehicle is tampered.

Selling, buying and moving with a diesel

Because diesel has no sale break, the seller usually needs a current smog certificate at transfer. Do the test within 90 days of the sale. A buyer should ask to see the smog report and can check the vehicle's history at bar.ca.gov/inspection.

If you move to California with a diesel, you need a smog check at first registration if the vehicle is in the program. California does not give a repair cost waiver for an initial registration of an out of state vehicle, so the vehicle must pass. Read the statistics on how often cars fail if you want the odds.