The EPA catalogs errors and corrections, as well as necessary revisions to test methods, performance specifications, and associated regulations in 40 CFR parts 51, 60, and 63, and periodically updates and revises these provisions. The most recent updates and revisions were promulgated on March 7, 2023 (88 FR 18396). This proposed action addresses necessary corrections and revisions identified after that final action, many of which were brought to our attention by regulated sources and end-users, such as environmental consultants and compliance professionals. These proposed revisions would improve the quality of data obtained and give source testers the flexibility to use newly approved alternative procedures if applicable. This action proposes corrections and updates to several test methods and performance specifications for air emission sources that are subject to 40 CFR parts 60 and 63. The proposed revisions include corrections to testing provisions that contain inaccuracies and errors, updates to outdated test methods, and additional language to clarify the requirements within these test methods. For example, the omitted constant, K2, for equation 4-2 would be added in Method 4; the definition of centroidal area would be updated in Method 7E; and an erroneous reference would be corrected in Performance Specification 2. The EPA develops these actions every two years to keep methods up-to-date and to ensure that compliance testing and monitoring are done correctly. The proposed revisions would improve the quality of data but do not impose new substantive requirements on source owners or operators.