Paccar will have to defend its MX-13 fuel injectors on the merits. A federal judge in Seattle has refused to throw out a proposed class action brought by 31 buyers and lessees of Kenworth and Peterbilt trucks who say deposits building up inside the injectors on EPA 2021 MX-13 engines left them with knocking, shaking engines and trucks that cut power without warning.
The Oct. 1 order from U.S. District Judge Robert S. Lasnik in 10th Gear LLC v. PACCAR turned on the argument the manufacturer needed most: that because the injectors are emissions-related hardware, the Clean Air Act preempts state-law warranty claims over them. The court rejected that on both express and implied grounds, reasoning that the federal emissions warranty is tied to emissions noncompliance rather than to a defect in an emissions part, and that asking whether Paccar broke its own contract is not a question for the EPA.
A narrow ruling, not a verdict
What survived is one claim: breach of the express Engine Warranty, which runs 24 months, 250,000 miles or 6,250 hours, whichever comes first. Lasnik did not find the engines defective, did not certify a class, and left open whether remedies such as a recall or a mandated injector design would be preempted. The injectors' emissions role, the order reasoned, does not convert every warranty obligation arising from their malfunction into a federal one.
Paccar denies breaching the warranty and says it supplied the required injector repairs and replacements at no cost to the plaintiffs.
The failure mode owners are describing
Deposits inside the injector are said to cause it to skip pilot injection, producing engine knock, heavy vibration, rough running and sudden derates. Land Line's account of the complaint traces the fixes attempted since production started in May 2021: dealers told in March 2022 to clean affected injectors — explicitly not a permanent repair — then fuel additives, software updates, a modified injector in April 2023, and finally the redesigned "Pack 2" injector, selectively available for model years 2021 to 2023 around June 2024.
The numbers in the filings are what fleets will recognize. Lead plaintiff 10th Gear says trouble started just past 1,000 miles and that it logged eight fuel-system incidents by roughly 206,600 miles. Little Diesel Transportation counted 19 repair visits and 110 days of downtime across five trucks as of February 2024. An internal Paccar email from October 2024 put roughly a third of trucks with the engine as having already had an injector repair. Paccar extended injector coverage to 10 years or 435,000 miles; plaintiffs argue that extension let the company avoid a recall while its repairs stayed temporary.
What to check before you pay for an injector job
If you run EPA 2021 MX-13 power, pull the unit history now rather than after the next derate. Every injector cleaning or replacement on those work orders is the paper trail that decides whether a repair you already paid for falls inside the extended 10-year, 435,000-mile injector coverage. Check which units are exposed: confirm which Kenworth and Peterbilt model years were built with the EPA 2021 MX-13 across your tractor and vocational fleet.
On the parts side, the question is whether what went in was the redesigned part or another of the original design. The invoice part number answers it, and it is worth cross-checking that injector number against its supersessions before you accept a quote — a same-design replacement buys you the same failure again.
Finally, do not assume knock-plus-derate is always injectors. Fuel quality, a failing pump and aftertreatment faults produce overlapping symptoms, and misdiagnosing one costs a set of injectors. Work the symptom back to a cause first, then order. And budget separately for downtime: the warranty at issue excludes lost income and consequential damages, which is precisely what the plaintiffs are fighting about.
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