The FIXER Act amends federal tax law to help finance the rehabilitation of aging affordable housing. The bill allows state and local authorities to issue tax-exempt bonds without counting them against their annual volume caps if the bonds are used to preserve, improve, or replace low-income residential buildings that are no longer receiving federal subsidies or have completed their original affordability periods. This exception particularly targets federally-assisted and state-assisted housing that needs renovation. The bill also modifies rules that typically restrict the use of tax-exempt bond proceeds for improvements to existing properties, raising the threshold from 15 percent to 50 percent for projects using these exempt bonds. The changes take effect immediately for bonds issued after the law is enacted, giving states and localities new flexibility to fund critical repairs and improvements to aging affordable housing stock without competing for limited bond financing capacity.