40 CFR §205.171-6

1. (a)The manufacturer of the exhaust system must conduct one valid test in accordance with the appropriate test procedure specified in appendix I for each exhaust system selected for testing under this subpart.
2. (b)No maintenance may be performed on the test exhaust system except as provided by § 205.171-2. In the event an exhaust system is unable to complete the noise emission test, the manufacturer may replace the exhaust system. Any replacement exhaust system must be a production exhaust system of the same category as the exhaust system which it replaced, and it is subject to all the provisions of this subpart.