The Magnusson Moss provisions are contained in Title 15 US Code Chapter 50 Section 2302 (c):

(c) No warrantor of a consumer product may condition his written or implied warranty of such product on the consumer's using, in connection with such product, any article or service (other than article or service provided without charge under the terms of the warranty) which is identified by brand, trade, or corporate name; except that the prohibition of this subsection may be waived by the Commission if -

(1) the warrantor satisfies the Commission that the warranted product will function properly only if the article or service so identified is used in connection with the warranted product, and

(2) the Commission finds that such a waiver is in the public interest.

The above section prohibits a warrantor from requiring use of purchased parts from the same source as the original equipment or some other specified source . It allows use of aftermarket purchased parts from other sources unless the Federal Trade Commission has granted to the warrantor a waiver of this section of the law.

The US Code of Federal Regulations provides additional guidance on warranties:

TITLE 16--COMMERCIAL PRACTICES
CHAPTER I--FEDERAL TRADE COMMISSION

PART 701--DISCLOSURE OF WRITTEN CONSUMER PRODUCT WARRANTY TERMS AND CONDITIONS--Table of Contents

Sec. 701.3 Written warranty terms.

(a) Any warrantor warranting to a consumer by means of a written warranty a consumer product actually costing the consumer more than $15.00 shall clearly and conspicuously disclose in a single document in simple and readily understood language, the following items of information:

(1) The identity of the party or parties to whom the written warranty is extended, if the enforceability of the written warranty is limited to the original consumer purchaser or is otherwise limited to persons other than every consumer owner during the term of the warranty;

(2) A clear description and identification of products, or parts, or characteristics, or components or properties covered by and where necessary for clarification, excluded from the warranty;

(3) A statement of what the warrantor will do in the event of a defect, malfunction or failure to conform with the written warranty, including the items or services the warrantor will pay for or provide, and, where necessary for clarification, those which the warrantor will not pay for or provide. (emphasis added)