CARB Compliance for Reefer Trailers: The Transport Refrigeration Unit (TRU) Rule

A refrigerated trailer has two separate power sources: whatever pulls it down the road, and the small diesel engine mounted on the nose that runs the refrigeration unit. California regulates that second engine — the Transport Refrigeration Unit, or TRU — under its own rule, completely separate from the aerodynamic-device requirement that applies to the trailer body. A reefer can be fully compliant on aero devices and tires and still be out of compliance on its TRU, so it's worth understanding as its own topic.

The Law: CARB's Transport Refrigeration Unit (TRU) Regulation

The rule is CARB's Airborne Toxic Control Measure (ATCM) for diesel-fueled Transport Refrigeration Units, first adopted in 2004 and significantly updated by amendments CARB approved in February 2022. It targets the diesel particulate matter these small engines put out, and it applies to any diesel-powered TRU that operates in California — including trailer-mounted units based anywhere else in the country. If a reefer trailer's TRU ever runs in the state, the rule reaches it regardless of where the trailer is registered or domiciled.

What's Actually Required

Every diesel TRU that operates in California — trailer, truck-mounted, or shipping container — has to be registered with CARB through the Air Resources Board Equipment Registration (ARBER) system and assigned a CARB identification number, even if the unit is based out of state. From there, the core technical requirement is the Ultra-Low-Emission TRU (ULETRU) in-use performance standard, which generally applies starting seven years after the TRU engine's model year and requires roughly an 85% reduction in particulate emissions compared to an uncontrolled engine.

On top of that, TRUs manufactured from December 31, 2022 onward must use a refrigerant with a global warming potential of 2,200 or less — older units running higher-GWP refrigerant like R-404A aren't automatically out of compliance, but it's a separate spec worth checking alongside the emissions side. Compliant units carry a CARB compliance label, and once a TRU is first reported to CARB, the owner has ongoing reporting obligations and pays a registration fee roughly every three years.

How a Trailer's TRU Reaches Compliance

There are a few paths to meeting the ULETRU standard. The simplest is a factory-built low-emission engine — newer TRUs generally ship compliant from the manufacturer with no modification needed. For an older unit, the usual route is a Level 3 Verified Diesel Emission Control Strategy (VDECS) retrofit — typically a diesel particulate filter added to the existing engine to bring it up to the required PM reduction, verified by CARB for that specific engine/TRU combination. A smaller number of fleets use alternative technology instead, such as electric standby or hybrid-electric systems, which can qualify in place of the diesel-only path but come with their own recordkeeping requirements.

Zero-Emission Requirements Don't (Yet) Apply to Trailers

CARB's 2022 amendments included a separate mandate requiring truck-mounted TRUs to go fully zero-emission by December 31, 2029 — but two things are worth knowing about that. First, it only ever applied to TRUs mounted on straight trucks, not to trailer, rail car, or shipping container units; the ATCM doesn't impose a zero-emission end point for reefer trailers at all. Second, that specific requirement is currently unenforceable regardless: U.S. EPA's Clean Air Act preemption waiver for the 2022 amendments, issued January 3, 2025, covered most of the rule but explicitly excluded the zero-emission TRU requirement, leaving it on hold pending further federal action.

Which Units Are CARB Compliant

There's no single public list of "compliant models" the way there is for SmartWay aerodynamic devices — compliance depends on the specific engine, its model year, and whether a retrofit has been applied, not just the make and model of the TRU. In practice, checking a specific unit means one of a few things: looking for the CARB compliance label physically affixed to the TRU, using the manufacturer's own compliance lookup tool by serial number (Thermo King and Carrier Transicold both offer these for their trailer units), or looking up the unit's CARB identification number in ARBER.

As a general rule, current-generation factory units — Thermo King's Precedent S-Series trailer line is a common example — ship compliant with no added equipment. Once a TRU engine passes roughly seven years past its model year, it's crossed into ULETRU territory, and that's the point where it matters most to confirm whether a Level 3 VDECS retrofit has actually been installed — age alone doesn't tell you either way.

Enforcement

CARB enforces this rule at facilities as well as on the road — refrigerated warehouses, distribution centers, grocery stores, seaports, and railyards that meet certain size thresholds have their own registration and quarterly reporting obligations for the TRUs that visit them. Penalties for non-compliance can run up to $10,000 per day, and they can land on the TRU owner, the facility operator, or both.

What This Means When You're Buying or Selling

If you're buying a reefer trailer for use in California, ask for the TRU's CARB ID number and compliance documentation rather than assuming compliance from the unit's age or a general "CARB compliant" claim in the listing. If you're selling, having that documentation — or clearly noting a VDECS retrofit if one's been done — answers the question a serious buyer is going to ask anyway.

Read our guide to CARB's aerodynamic device requirements for dry vans and reefers →

This Isn't Legal Advice

This article is a general summary for buyers and sellers, not legal or regulatory advice, and CARB's TRU rules, deadlines, and enforcement practices can and do change. If you're unsure whether a specific unit or use case is compliant, confirm directly with CARB or a qualified transportation compliance professional before making a decision based on anything here.