California : Air Resources Board Proposes to Align Truck Engine and Fuel Rules with Federal Standards

As of model year 2027, California plans major updates to truck engine and fuel rules.
These include tighter emission standards for heavy-duty engines, changes to on-board diagnostic systems, revised warranty and durability rules, and new corrective action procedures.

Emergency vehicle emission rules will become permanent to ensure sales continue despite federal challenges.
Changes also extend to the Low Carbon Fuel Standard, adding new crediting for renewable energy in vehicle charging.
The rules affect engine manufacturers, transit agencies, fuel suppliers, and fleet operators.

California is proposing several regulatory changes that will apply mainly to medium- and heavy-duty engine manufacturers, fuel providers, and vehicle operators beginning in 2027.

Engine and emission standards

• California will align most heavy-duty engine rules for model year 2027 and beyond with federal standards.
• Updates cover certification testing, averaging and credit programs, warranty and durability periods, and in-use testing methods.
• The exemption process for diesel bus engines at transit agencies will end in 2027.

On-board diagnostics (OBD)

• Definitions of testing cycles will be updated, and aging requirements for engines will change.
• New, higher thresholds will apply to identify failures, with more flexible testing criteria.
• Additional data stream parameters will be required on heavy-duty engines.
• Monitoring criteria for NOx sensors, particulate filters, and catalyst systems will be revised to improve reliability.
• Manufacturers will face new standardized reporting templates and adjusted deadlines.
• The rules delay some requirements for light- and medium-duty vehicles, giving industry more time.

Emission warranty and corrective action procedures

• For vehicles from 2027 onward, recall triggers and corrective actions will change.
• Proof of correction for software updates will no longer need physical repair labels.
• Manufacturers may use alternative ways to collect owner information during recalls.

Emergency vehicle rules

• Emergency vehicle emission standards introduced during recent federal disputes will become permanent.
• This ensures sales can continue in California even if federal courts or Congress challenge California’s waiver authority.
• Manufacturers may choose to certify under either the latest rules or older pre-2027 rules, but risk non-compliance if legal challenges are overturned.

Low Carbon Fuel Standard (LCFS)

• California will allow indirect accounting of renewable natural gas used in linear generators that produce electricity for vehicle charging.
• This provides more near-term options for truck charging with lower carbon intensity.

Timeline and enforcement

• Most changes apply to engines and vehicles starting with model year 2027.
• CARB, the California Air Resources Board, is the authority responsible for enforcement.
• Companies that fail to comply may face recalls, corrective actions, or penalties.

Reason for changes

• California seeks to align with federal standards, reduce emissions, and provide clarity during federal legal disputes.
• The LCFS update supports faster deployment of low-carbon truck charging.

Compliance resources

• Detailed CARB guidance and rulemaking updates are available here: https://ww2.arb.ca.gov.

Sources :

Titles 13 AND 17, Air Resources Board, On-Road Heavy-Duty Engine and Vehicle Omnibus, Low Carbon Fuel Standards Regulations, and to Permanently Adopt Emergency Vehicle Emissions Regulations, Vol. No. 39-7 California Register (September 26, 2025)