The Engine Family Number (EFN) is a 10-12 numeric code that can be found on the Emission Control Label attached to your vehicle. Typically, the number is found on the actual underside of the hood, on the door, or on the fender well of the Truck.

Here are some examples:

source: https://arch.umd.edu/sites/default/files/docs/Locating the EPA Engine Family Name.pdf

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