EPA Engine Family Name: Code Format, Location, and Compliance

An EPA engine family name is a 12-character alphanumeric code the U.S. Environmental Protection Agency uses to group engines with similar designs and emission profiles into a single “family” for Clean Air Act certification. Rather than testing every individual engine that rolls off a production line, the EPA certifies a representative sample and ties every unit back to that certification through this code. Each of the 12 positions carries a specific piece of information: the model year, the manufacturer, the type of engine, its technical characteristics, and a manufacturer-defined suffix that separates configurations within the same family.¹

How to Read the 12 Characters

The code is read left to right, and each position answers a different question about the engine. Once you know what each slot represents, a string that looks like gibberish becomes a compact record of the engine’s regulatory identity.

Position 1 — Model year. A single letter or digit. For 2026, the character is “T.”
Positions 2 through 4 — Manufacturer code. A three-character code the EPA assigns to each company. This is what ties the engine back to a specific corporate entity responsible for compliance.
Position 5 — Industry sector. One character showing the regulatory category the engine falls into, such as light-duty vehicle, heavy-duty diesel, or highway motorcycle.
Positions 6 through 9 — Engine displacement or family descriptor. Four characters describing displacement or another distinguishing technical trait.
Positions 10 through 12 — Manufacturer-defined code. Three characters the manufacturer chooses to separate configurations that share the same year, sector, and displacement.

So a code that begins “TGMC” tells you the engine is a 2026 model year (T) from the manufacturer holding the code GMC, with the fifth character then telling you whether it’s a light-duty vehicle, a heavy-duty engine, or something else. The EPA publishes sector-specific instructions for positions 6 through 12 on its certification website, because what those characters mean varies by engine category.¹

Model Year Codes

The first character cycles through the alphabet, skipping “I” and “O” to avoid confusion with the digits 1 and 0. After “Y” (2030), the sequence rolls to numeric digits for 2031 through 2039 and then repeats. Recent and upcoming years:

R — 2024
S — 2025
T — 2026
V — 2027
W — 2028
X — 2029
Y — 2030

The EPA keeps the full historical table going back to 2001 on its family naming page.¹

Industry Sector Codes

Position 5 identifies which regulatory category the engine belongs to, and each category is measured against its own emission standards. Some of the most common codes:

V — Light-duty vehicles
T — Light-duty trucks and medium-duty passenger vehicles
J — Combined light-duty vehicles and light-duty trucks
H — Heavy-duty highway diesel (compression-ignition) engines over 8,500 pounds GVWR
E — Heavy-duty highway gasoline (spark-ignition) engines over 8,500 pounds GVWR
C — Highway motorcycles
L — Nonroad compression-ignition engines
S — Small nonroad spark-ignition engines under 19 kilowatts
N — Marine compression-ignition engines

The complete list runs to more than 20 categories, including snowmobiles (Y), freshly manufactured locomotives (G), and remanufactured locomotive systems (K).¹ The character matters because it tells regulators which set of emission standards the engine was certified against. A code with the wrong sector letter points at the wrong rulebook.

Where the Engine Family Name Appears

Every certified engine has to carry a permanent label showing the engine family name. Federal rules require that label to stay legible for the engine’s entire operational life, to be attached to a part needed for normal operation, to be written in English, and to remain readily visible after installation. It also has to be secured so it cannot be removed without being destroyed or defaced.²

On most passenger vehicles, the label sits under the hood, typically on or near the valve cover, the radiator support, or the underside of the hood itself. Heavy-duty and nonroad engines usually carry the label on the engine block or another prominent external surface. Manufacturers use metallic plates or heat-resistant adhesive labels that can survive engine-compartment temperatures. Inspectors and technicians use the label to confirm that the physical emission hardware on a vehicle matches what the manufacturer certified.

What the Code Ties Back To

Behind every engine family name is a Certificate of Conformity, the document the EPA issues after reviewing a manufacturer’s certification application and confirming that the family meets Clean Air Act emission limits throughout the engine’s useful life. The certificate legally authorizes sale of engines in that family for the named model year.³

A detail worth knowing: engines produced after December 31 of the calendar year that names the model year are not covered by the original certificate, even if they are physically identical to units built earlier. A new certificate meeting current standards is required for those units.³

The engine family name is what threads a specific engine on the road back to the certificate that authorized its sale. If a label is missing, defaced, or shows a family name the EPA never certified, the underlying legal cover is gone. Selling a new engine or vehicle without a valid certificate is prohibited under the Clean Air Act, and civil penalties assessed in 2025 or later reach up to $59,114 per engine or vehicle.⁴

Tampering With a Certified Engine

Because the engine family name identifies a specific certified configuration, altering the emission hardware on a labeled engine puts the vehicle out of step with the certificate the label points to. The Clean Air Act makes it illegal to remove or disable any emission control device or design element installed to meet federal standards, both before and after sale to the end buyer.⁵ That covers physically removing a catalytic converter, and it also covers running a diesel engine without the correct quality of diesel exhaust fluid when the emission system depends on it.

Manufacturing, selling, or installing aftermarket parts whose principal effect is to bypass or disable emission controls is separately prohibited. Civil penalties differ by who is doing the tampering. For a manufacturer or dealer, the cap reaches $44,539 per engine or piece of equipment. For anyone else, it is $4,454 per unit.⁶

There is one narrow exception. Aftermarket conversions, such as converting a gasoline engine to run on compressed natural gas, can qualify for an exemption if the converter can show the modified engine still meets applicable emission standards. A “new or relatively new” engine, meaning one within one year of its original model year, has to satisfy both intermediate and full useful life standards using appropriate deterioration factors. An engine that has already exceeded its useful life only has to show that emissions are maintained or improved after conversion.⁷

Using the Code to Verify Compliance

Once you can read the label, you can check whether an engine’s on-paper identity lines up with its physical hardware. The model year character should match the vehicle’s build date. The sector character should match the vehicle type: a light-duty passenger car should not carry a code that starts its fifth position with “H” for heavy-duty diesel. The manufacturer code should trace to the company that actually built the engine. Mismatches are a signal that something has been swapped, altered, or mislabeled, and any of those situations puts the engine outside the certificate that authorizes its sale and use.

For most drivers and buyers, the engine family name is a lookup tool. For manufacturers, importers, and shops working on certified engines, it is the anchor point for a much longer chain of Clean Air Act obligations, from production records to in-use testing to recalls when defects appear. Whichever side of that chain you are on, the 12 characters on the label are where it all starts.

1
U.S. Environmental Protection Agency. Information about Family Naming Conventions for Vehicles and Engines
2
eCFR. 40 CFR 1068.45 – General Labeling Provisions
3
eCFR. 40 CFR 85.2305 – Duration and Applicability of Certificates of Conformity
4
eCFR. 40 CFR 19.4 – Statutory Civil Monetary Penalties, as Adjusted for Inflation, and Tables
5
Office of the Law Revision Counsel. 42 USC 7522 – Prohibited Acts
6
eCFR. 40 CFR 1068.101 – What General Actions Does This Regulation Prohibit?
7
eCFR. 40 CFR 85.505 – Overview