I like this section of that link:
Aftermarket parts or modifications:
This aspect of warranty coverage has a great deal of gray area. Although many dealers would have you think otherwise, simply having an aftermarket part or modifying your vehicle cannot void your warranty.
"Some dealerships may say 'just because you have a [cold air] intake or something' that the whole vehicle warranty is voided," explains Loren Wong, Edmunds associate business analyst and a former warranty administrator for BMW and Acura. "That's not true."
The saving grace for consumers is the Magnuson-Moss Warranty Act of 1975
. The act states that a dealer must prove that aftermarket equipment caused the need for repairs before it can deny warranty coverage.
However, if the reason for a parts failure is unclear, a dealer will usually charge you to diagnose the vehicle. If the aftermarket part was not properly installed or a modification led to a component failure, it is within the dealer's right to void the warranty for that part, and you will have to pay for the repairs out of pocket. If the aftermarket parts had nothing to do with the repairs in question, you will be refunded the fee for the diagnosis.
Any aftermarket performance parts on your vehicle can cause a dealer to suspect that you either drive the car hard or possibly race it. "Although they may not void warranties," Wong added, "modifications may raise a red flag when vehicles are in for service. If consumers who mod their cars do a little research, they may find certain dealerships that are a little more 'mod-friendly.'"