No dealer can void the whole vehicle's warranty. If you modify a stock part, of course it won't be under warranty anymore. If you modify (or remove) your stock air box, it will no longer be under warranty. However, the dealer can not void the warranty of anything else "down the line" from the mod that you installed.

For instance, if you replace your stock air box with a high-flow intake with a K&N Filtercharger cone filter, the dealer can't void the warranty on your throttle body, O2 sensor, or intake manifold.

Of course, there will always be exceptions to the rule. You may want to check with your dealer before you do a mod, just to be safe. If you REALLY want to be safe, wait until your vehicle has hit 3 years or 36,000 miles, or whatever mileage/year combination the vehicle manufacturer's warranty covers.

For more information on what to do if the dealership refuses to honor your warranty, contact SEMA (Speciality Equipment Market Association) at http://www.sema.org/main/semaorghome.aspx?ID=50096
If you have installed a reputable after-market part, SEMA may be able to assist you in getting that warranty issue taken care of.

For information on the Magnuson-Moss Warranty Act, visit: