The following restrictions apply for using emission credits:

(a) Averaging sets. Except as specified in paragraph (c) of this section, emission credits may be exchanged only within the following averaging sets based on primary intended service class:

(1) Spark-ignition HDE.

(2) Light HDE.

(3) Medium HDE.

(4) Heavy HDE.

(b)-(c) [Reserved]

(d) Credit life. NOX credits may be used only for five model years after the year in which they are generated. For example, credits you generate in model year 2027 may be used to demonstrate compliance with emission standards only through model year 2032.

(e) Other restrictions. Other sections of this part specify additional restrictions for using emission credits under certain special provisions.

[88 FR 4487, Jan. 24, 2023, as amended at 91 FR 7782, Feb. 18, 2026]