40 CFR 91.117 - Certification procedure
(1) Any engine required to be tested under § 91.118 must be operated with all emission control systems operating properly for a period sufficient to stabilize emissions prior to such testing.
(2) A manufacturer may elect to consider emission levels as stabilized when the test engine has accumulated 12 hours of service.
(b) No maintenance, other than recommended lubrication and filter changes, may be performed during service accumulation without the Administrator's approval.
(c) Service accumulation is to be performed in a manner using good engineering judgment to ensure that emissions are representative of production engines.
(d) The manufacturer must maintain, and provide to the Administrator if requested, records stating the rationale for selecting a service accumulation period different than 12 hours and records describing the method used to accumulate hours on the test engine(s).