30 CFR § 57.5066
(a) Any diesel powered equipment operated at any time in
underground areas must meet the following maintenance standards:
(1) The operator must maintain any approved engine in approved
(2) The operator must maintain the emission related components of
any non-approved engine to manufacturer specifications; and
(3) The operator must maintain any emission or particulate control
device installed on the equipment in effective operating condition.
(b)(1) A mine operator must authorize each miner operating diesel-powered equipment underground to affix a visible and dated tag to the equipment when the miner notes evidence that the equipment may require maintenance in order to comply with the maintenance standards of paragraph (a) of this section. The term "evidence" means visible smoke or odor that is unusual for that piece of equipment under normal operating procedures, or obvious or visible defects in the exhaust emissions control system or in the engine affecting emissions.
(2) A mine operator must ensure that any equipment tagged pursuant to this section is promptly examined by a person authorized to maintain diesel equipment, and that the affixed tag not be removed until the examination has been completed. The term "promptly" means before the end of the next shift during which a qualified mechanic is scheduled to work.
(3) A mine operator must retain a log of any equipment tagged
pursuant to this section. The log must include the date the equipment
is tagged, the date the equipment is examined, the name of the person
examining the equipment, and any action taken as a result of the
examination. The operator must retain the information in the log for
one year after the date the tagged equipment was examined.
(c) Persons authorized by a mine operator
to maintain diesel equipment covered by paragraph (a) of this section must be
qualified, by virtue of training or experience, to ensure that the maintenance
standards of paragraph (a) of this section are observed. An operator must retain
appropriate evidence of the competence of any person to perform specific
maintenance tasks in compliance with those standards for one year after the date
of any maintenance, and upon request must provide the documentation to the
authorized representative of the Secretary.