The Federal Trade Commission warned six companies today that their warranty policies could be in violation of the law. Although we don’t know the identities of these companies, the FTC does note that they’re “major companies” that sell cars, cellphones, and video game systems. The companies currently claim their warranties are void if owners use them with unauthorized parts or services or if consumers open them up by removing a seal.

Companies can’t put warranty stickers on their products and claim the warranty is void if you remove the sticker, the FTC says. They also can’t void a warranty if you fix devices yourself or through a third-party service. This has been a contentious issue for consumer electronics, where it’s often hard to get repairs done through a third party. Apple, for instance, recently said that third-party screen repairs would void an iPhone’s warranty. It changed its policy in February so that those phones are eligible for partial warranty coverage.

The FTC warning states that language like “this warranty shall not apply if this product ... is used with products not sold” and requiring warranty stickers to maintain a warranty are all illegal. The commission requests that each of the six companies review their warranty materials and correct their policies within 30 days. If they don’t comply, “potential violations may result in law enforcement action.”