New California Smog Check Rules 2023 (Diesel Cars)

Attention all 2007 and newer light-duty diesel vehicle owners in California: as of July 2023, the Golden State is implementing new Smog Check requirements that you need to know about.

These changes, introduced by the Bureau of Automotive Repair (BAR), are aimed at reducing diesel emissions and ensuring compliance with clean air goals.

Key Takeaways

  • Starting July 2023, California is implementing new Smog Check requirements for light-duty diesel vehicles to reduce diesel emissions and meet clean air goals.
  • The new requirements will affect light-duty diesel vehicles with a model year of 2007 or newer.
  • The new requirement concerns the “readiness monitors”.
  • Vehicle owners should be aware of potential exemptions that may apply based on design-related issues impacting emission performance.

Overview Of California’s New Diesel Smog Check Requirements For Light-Duty Vehicles

Starting in July 2023, the state of California will be implementing new Smog Check requirements for light-duty diesel vehicles, which are those with a gross vehicle weight rating (GVWR) of less than 14,001 pounds.

The new smog check requirements introduce a new rule regarding which readiness monitors can be left unset.

The Importance Of Readiness Monitors For A Smog Check Inspection

Since 2010, all light-duty diesel vehicles with model year 1998 and newer have been subject to smog checks.

An important part of the smog check is to run an assessment of the vehicle’s On Board Diagnostics (OBD) system to ensure that the readiness monitors are all running correctly. The readiness monitors are referred to as being “run and set”.

Light-duty vehicles with model year 1998 to 2006 must have all their readiness monitors set for the inspection.

However, due to added complexity of light-duty vehicles made from 2007 and onwards, these vehicles have been allowed to leave two readiness monitors unset.

The new requirements introduce a new rule with regards to these unset readiness monitors.

How The New Smog Check Rule Affects Your Readiness Monitors

This new rule specifies which readiness monitors are allowed to be unset.

Only the two readiness monitors that are specified in the rule are allowed to be unset, as opposed to the previous situation in which any two readiness monitors could be left unset.

The two readiness monitors which are allowed to be unset in a light-duty diesel vehicle smog check are:

  1. The Diesel Particulate Filter
  2. The Non-Methane Hydrocarbon Catalyst

For example, if you own a light-duty diesel vehicle from model years 1998-2006, all readiness monitors must be set for your car to pass the Smog Check inspection.

On the other hand, newer models from 2007 onwards will only be allowed two unset readiness monitors – specifically Diesel Particulate Filter (DPF) and Non-Methane Hydrocarbon Catalyst (NMHC Catalyst).


Applicable Vehicles By Model Year

It is important to know which vehicles are included in the new smog check requirements to ensure compliance and avoid penalties. Here are the details on which diesel-powered light-duty vehicles will be affected by the updated regulations:

Model YearApplicable VehiclesNotes
1998 – 2006Light-duty diesel vehicles with a gross vehicle weight rating of less than 14,001 poundsThese vehicles must have all readiness monitors set to pass the smog check inspection.
2007 – 2022Light-duty diesel vehicles with a gross vehicle weight rating of less than 14,001 poundsThese vehicles can have up to two unset readiness monitors due to the complex exhaust after-treatments added in 2007.
Starting July 2023Model year 2007 and newer light-duty diesel vehicles with a gross vehicle weight rating of less than 14,001 poundsOnly two specific monitors will be allowed to be unset as per the updated standards.
ExemptionsSome vehicles may be exempt from these requirements due to design-related issues.The BAR On-Board Diagnostic Inspection System will automatically identify and exempt these vehicles.

Make sure to check the On-Board Diagnostic Test Reference webpage at for the readiness standards for OBD tests during a Smog Check inspection and the list of exempted vehicles.


As a light-duty diesel vehicle owner, it’s essential to be aware of the new California Smog Check requirements and potential exemptions that may apply to your vehicle. Some significant exemptions include:

  1. Vehicles with design-related issues that prevent them from meeting the new monitor readiness standards will be exempted from the updated Smog Check requirements.
  2. The Bureau of Automotive Repair (BAR) On – Board Diagnostic Inspection System will automatically identify these exempted vehicles during the Smog Check inspection.
  3. This exemption will remain in place until manufacturers correct the design issues impacting their emission performance.


What are the new diesel smog check requirements for light-duty vehicles in California starting 2023?

Starting in 2023, light-duty diesel vehicles (vehicles weighing up to 14,000 pounds) with model year 2007 or newer, will only be allowed to have specific readiness monitors unset in their smog inspection. Previous to the date of the new requirements, any two readiness monitors could be left unset.

Will these new requirements apply to all diesel-powered light-duty vehicles regardless of their age?

No, these requirements will only apply to all light duty diesel vehicles with model year 2007 or newer. Some vehicles will be exempt temporarily until the manufacturer addresses certain design issues.

How often do diesel light duty vehicles need to undergo a smog check under the new requirements?

The new smog check requirements concerning diesel-powered light duty vehicles, does not concern the frequency that an inspection is required. The frequency of inspection remains the same as it previously was, i.e. biennially, or in other words, every other year.

What happens if my vehicle fails its smog check?

If your vehicle fails its initial smog check, you’ll be directed by the testing facility towards repair options that can make your engine more efficient and emit fewer pollutants.

Once repaired, you may be entitled for a re-test at no additional cost depending on certain criteria set forth by the testing facility and/or local agency overseeing compliance with emission standards in your area.

Jonathan Rice

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