K&N Seeks Declarations That Its NextGen Cold Air Intake Does Not Infringe Advanced Flow Engineering's Air Filter Patent and That the Patent Is Invalid.

The parties and the court

The case is K&N Engineering, Inc. v. Advanced Flow Engineering, Inc., No. 5:26-cv-06177 (C.D. Cal.), filed October 19, 2026. The complaint says K&N is a California corporation based in Southern California, and alleges, on information and belief, that Advanced Flow Engineering is a California corporation based in Corona, California. K&N seeks declaratory judgments of noninfringement and invalidity of U.S. Patent No. 9,440,175 under the Declaratory Judgment Act, 28 U.S.C. 2201 and 2202, and demands a jury. Venue is pleaded under 28 U.S.C. 1391(b) and (c) and 1400(b). K&N is represented by Stradling Yocca Carlson & Rauth LLP.

The cease and desist letter

The complaint says that on September 10, 2026, Advanced Flow Engineering sent K&N a cease and desist letter accusing the NextGen Cold Air Intake System, Model No. 50-9039, of infringing at least claims 1 and 2 of the '175 patent. According to the complaint, the letter threatened litigation seeking an injunction, lost profits, reasonable royalties, enhanced damages for willful infringement, and attorneys' fees under 35 U.S.C. 284 and 285 unless K&N took a license, changed its design, or stopped sales within 30 days. K&N pleads that letter as the basis for an actual controversy under 28 U.S.C. 2201.

The '175 patent

U.S. Patent No. 9,440,175, titled "Interface Air Filter and Assembly," issued September 13, 2016 from Application No. 13/694,159, filed November 1, 2012. It has four claims, and claims 1 and 2 are independent. Google Patents lists an adjusted expiration date of November 20, 2033.

Claim 1 covers an interface air filter for an internal combustion engine comprising an air filter with a closed end and an open end and a reducing coupling whose body fixedly connects a first and a second engaging member. The coupling's first engaging member has an outer surface with a plurality of circumferential ridges extending outwardly to engage an air intake port of a housing, and is fixedly attached to the open end of the filter so that its outer surface extends outwardly from the filter. The second engaging member has a smaller external circumference, an inner surface that engages the outer surface of an air intake tube, and an end adapted to receive a clamp.

Claim 2 recites a similar filter and coupling together with a non-porous housing having an air intake port and an opening, with the filter inside the housing and the first engaging member disposed in the housing opening in contact with the housing in a substantially air tight manner. Claim 3 adds circumferential ridges extending inwardly from the inner surface of the second engaging member, and claim 4 requires the non-porous housing to be a molded single piece structure.

Noninfringement theory

K&N alleges that Model No. 50-9039 lacks the "plurality of circumferential ridges extending outwardly from the outer surface" that every independent claim requires, and that it uses a smooth-surfaced coupling instead. The complaint includes an excerpt of Figure 6 of the patent and a photograph of the product that it says shows an outer surface without any ridges.

On the doctrine of equivalents, the complaint relies on prosecution history estoppel. It alleges that during prosecution the examiner rejected the claims under 35 U.S.C. 102 and 103 over references including U.S. Patent No. 7,645,329, US 2006/0260469 (Miyagishima), U.S. Patent No. 5,275,636 (Dudley), and U.S. Patent No. 5,487,767 (Brown), and that the applicant amended claims 1 and 2 to add the outward circumferential ridges and argued that Brown's ribs are interior while the claims require exterior ridges.

Invalidity theory

The second count alleges invalidity under 35 U.S.C. 101, 102, 103, and 112. The complaint's obviousness theory pairs Miyagishima or Dudley with Brown, and it quotes the patent's own statements that ridges on the outer surface of the first engaging member and on the inner surface of the second engaging member each "increase the contact effectiveness" (col. 3, lines 28 to 29 and 64 to 67). It also cites Japanese Patent Publication No. H08-42767, U.S. Patent No. 3,464,721 (Surko), and EP 1024875 as teaching circumferential ridges on flexible sealing members. The section 112 theory invokes written description, enablement, and definiteness: the complaint alleges that the patent does not let a skilled artisan understand the term "ridges" with reasonable certainty or implement such ridges without undue experimentation.

Relief sought

K&N asks for declarations that it and its officers, agents, distributors, and customers do not infringe any claim of the '175 patent and that each claim is invalid; a preliminary and permanent injunction barring Advanced Flow Engineering from asserting or threatening infringement claims on the patent against K&N or its customers; and a finding that the case is exceptional under 35 U.S.C. 285 with an award of attorneys' fees, costs, and expenses.

What to watch next

Watch for Advanced Flow Engineering's answer, including whether it files infringement counterclaims on claims 1 and 2, or a motion to dismiss. Also watch for the scheduling order and any early claim construction dispute over "plurality of circumferential ridges extending outwardly from the outer surface."