40 CFR 90.204 - Averaging.
(a) Negative credits from engine families with FELs above the applicable emission standard must be offset by positive credits from engine families having FELs below the applicable emission standard, as allowed under the provisions of this subpart. Averaging of credits in this manner is used to determine compliance under § 90.207(b). A manufacturer may have a negative balance of credits as allowed under § 90.207(c)(2).
(c) Credits used in averaging for a given model year may be obtained from credits generated in the same model year by another engine family, credits banked in previous model years, or credits of the same or previous model year obtained through trading subject to the provisions of § 90.205(a). Credits generated under the previously available “Optional transition year averaging, banking, and trading program for Phase 2 handheld engines” of §§ 90.212 through 90.220, since repealed, may also be used in averaging. The restrictions of this paragraph notwithstanding, credits from a given model year may be used to address credit needs of previous model year engines as allowed under § 90.207(c).