Dealerships will lie to their customers simply to try and force them to only come back to them for repairs, maintenance, and upgrades. Most of the time the dealerships will actually contact aftermarket shops to do work for them to close a sale on a new or used vehicle. This includes window tint, remote starts/security systems, headrest monitors, backup cameras and more.
The Magnuson-Moss act states that aftermarket parts and installs WILL NOT VOID your warranty unless the installation/part is the direct cause of the failure. Then the warranty is only void on that particular failure in question and not the entire vehicle warranty. This is why it is very important to have professional installers doing the work for you to minimize any chances of something being installed improperly.
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Taken from Directed Electronics TechTips 1003:
Dear Consumer: Using Viper or other quality brand name auto security products does not void vehicle warranties.
Consumers who purchase auto security systems are sometimes told by an automobile dealer’s service representative or mechanic that after-market parts or products will “void the warranty” with the implication that only the car manufacturer’s own brand may be used.
This, of course, tends to cast doubt on the quality of the after-market products. This claim is simply and absolutely untrue.
Under the Magnuson-Moss Warranty Improvement Act, a vehicle manufacturer may not make its vehicle warranty conditional on the use of any brand of security system unless the manufacturer provides the system free of charge or the Federal Trade Commission (FTC) has specifically published that only vehicle manufacturer’s product may be used.
To challenge a false claim, ask the person to put it in writing or request the vehicle manufacturer security system free of charge. If you are charged for the security system, or they refuse to give you a written statement, there may be a violation of Federal law.