40 CFR § 85.2105 - Aftermarket parts.
(a) No valid emission performance warranty claim shall be denied on the basis of the use of a properly installed certified aftermarket part in the maintenance or repair of a vehicle. A vehicle manufacturer that honors a valid emission performance warranty claim involving a certified aftermarket part may seek reimbursement for reasonable expenses incurred in honoring the claim by following the warranty claim procedures listed in § 85.2107(c).
(b) Except as provided in § 85.2104(h), a vehicle manufacturer may deny an emission performance warranty claim on the basis of an uncertified aftermarket part used in the maintenance or repair of a vehicle if the vehicle manufacturer can demonstrate that the vehicle's failure to meet emission standards was caused by use of the uncertified part. A warranty claim may be denied if the vehicle manufacturer submits a written document to the vehicle owner that the vehicle owner is unable or unwilling to refute. The document must:
(1) Establish a causal connection between the emissions short test failure and use of the uncertified part, and,
(2) Assert that:
(i) Removal of the uncertified part and installation of any comparable certified or original equipment part previously removed or replaced during installation of the uncertified part will resolve the observed emissions failure in the vehicle, and/or
(ii) Use of the uncertified part has caused subsequent damage to other specified certified components such that replacement of these components would also be necessary to resolve the observed vehicle emissions failure, and,
(c) A part not required to be replaced at a definite interval in accordance with the written instructions for maintenance and use shall be warranted for the full term of any warranty mandated by the Act. Instructions to replace a component only if checked and found to be operating below specification shall have no bearing on warranty coverage, unless an owner did not follow such an instruction prior to the short test failure and noncompliance with that instruction caused the failure of another vehicle component relevant to the nonconformity.