Voiding a warrenty

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A few comments. One, a dealer cannot void a warranty, the dealer does not issue the warranty, the warranty is issued by GM. A GM rep can void the warranty, or refuse to service a part; however, the part has to have been damaged by a non-factory authorized component.
The Magnuson Moss Warranty Act gives you protection from items like air filters, exhaust, etc.
Also, if it is a legitimate reason for a manufacturer to void a warranty on a particular item, it does not mean the entire warranty is voided.
For example, you install monster tires with a radical backspace, and the tires stick out causing premature wear to the wheel bearings, the warranty on the bearings could be voided, but the remaining warranty is still good.
Another example, a supercharger will most likely void the engine/trans/differential warranty; however it will not affect the A/C system, brakes, radio, etc.
In addition to voiding a warranty, if you add something to the vehicle, and the dealer has to remove this item to service a warranty item, they can charge you for the time to remove the item. Also, a dealer can refuse to work on a vehicle if the modification is considered dangerous to the technician or the car. Example of this is many dealers will not position a lowered Vette onto their hoist since they are afraid of damaging the vehicle; even using wood blocks to drive the car onto the hoist.