Diesel Emissions Reduction Act of 2010

Diesel Emissions Reduction Act of 2010 - Amends the Energy Policy Act of 2005 to reauthorize and extend funding for a grant program for reducing diesel emissions. Authorizes the Administrator of the Environmental Protection Agency (EPA) to: (1) provide rebates, on a competitive basis, including through contracts for the administration of programs for providing rebates and loans, to eligible entities to achieve significant reductions in diesel emissions; and (2) support rebate programs administered by states that are designed to achieve significant reductions in diesel emissions.

Includes among entities eligible to receive funding for reducing diesel emissions any private individual or entity that: (1) is the owner of a diesel vehicle or fleet operated pursuant to a contract, license, or lease with a federal agency or a regional, state, local, or tribal agency or port authority with jurisdiction over transportation or air quality; and (2) meets such requirements as the Administrator may establish for vehicle use and for notice to and approval by such agency with respect to a contract, license, or lease.

Redefines "emerging technology" to mean a technology which is not or has not been certified or verified by, but for which an approved application and test plan has been submitted to, the Administrator or the California Air Resources Board.

Includes Puerto Rico, Guam, the Virgin Islands, American Samoa, and the Northern Mariana Islands within the meaning of "state" along with states and the District of Columbia.

Revises provisions concerning the distribution and use of funds and applications for such funding. Requires the Administrator to develop a simplified application process for applicants to expedite the provision of funds. Requires each state to give priority to projects that meet specified criteria. Requires the Administrator and each state to publish on its website the total number and dollar amount of rebates and loans provided, as well as a breakdown of the technologies funded and a description of each application for which a grant or loan is provided.

Requires the Administrator to include in a report on the implementation of such program sent to Congress before January 1, 2016, an analysis of the need to continue the program, including an assessment of the size of the vehicle and engine fleet that could provide benefits from being retrofitted under this program and a description of the number and types of applications that were not granted in the preceding year.

Requires the Comptroller General to carry out and report on an audit to identify: (1) all federal mobile source clean air grant, rebate, or low cost revolving loan programs under the authority of the Administrator, the Secretary of Transportation, or other relevant federal agency heads that are designed to address diesel emissions from, or reduce diesel fuel usage by, diesel engines and vehicles; and (2) duplication or overlap among, or gaps between, federal mobile source clean air programs.

Rescinds all unobligated amounts provided to carry out the pilot program for mailings of postal patron postcards by Senators for the purpose of providing notice of town meetings the Senator will attend.