It's in Writing, and it's Right in Your GloveBox
Every person who buys a new Mustang receives a Warranty Guide booklet, and it's likely buried amongst your registration card, insurance card, and extra Wendy's ketchup packets. Reading it will reveal a great deal of information regarding Ford's legal obligations. Here are a few of the more important sections, with emphasis added.
Ford states that its warranty will not cover "damage caused by . . . misuse of the vehicle, such as driving over curbs, overloading, [or] racing . . . " Note that this does not say that Ford's warranty will not cover vehicles used for racing; it says it won't cover damage caused by racing. One should not expect Ford to warrant one's driveshaft when the owner slaps slicks on the car, takes it to a dragstrip, and drops the clutch at six grand-while spraying a 175-shot of nitrous. But that same person would want Ford to repair that same "used-for-racing" car when the weatherstripping on the driver-side window springs a leak.
Here's a key section: "The New Vehicle Limited Warranty does not cover any damage caused by:Alterations or modifications of the vehicle, including the body, chassis, or components, after the vehicle leaves the control of Ford Motor CompanyTampering with the vehicle, tampering with the emissions systems or with the other parts that affect these systems (for example, but not limited to exhaust and intake systems)The installation or use of a non-Ford Motor Company part (other than a "certified" emissions part) or any part (Ford or non-Ford) designed for "off-road use only" installed after the vehicle leaves the control of Ford Motor Company, if the installed part fails or causes a Ford part to fail. Examples include . . . performance-enhancing powertrain components."
There are a few things to talk about here. First off, the entire section is referring to damage "caused by" modifications. Damage "caused by" tampering with the intake system would presumably refer to the modification adversely affecting, for example, sensors such as the MAF and IAT, and damage "caused by" tampering with the exhaust system likely refers to, for example, damage to the oxygen sensors.
One Mustang owner found himself in a real fiasco when a dealership said his engine problem (from his description, a valvetrain tick) was the result of his cold-air intake sucking in water and "rust getting up in there." In light of the facts that plastic intake tubes, aluminum intake manifolds, and aluminum cylinder heads don't rust, the dealership personnel clearly were barking up the wrong tree as to causation here. Even more telling, the dealer hadn't even disassembled the engine, and yet had managed to reach the precise diagnosis that it was "all shot ta hell, ya know!"
The last bullet point in the above-quoted text refers to non-Ford parts failing or "causing" a Ford part to fail. Of course we don't expect Ford to replace our rusted-out Brand-X headers under warranty, as they're clearly a non-Ford part. What we do expect Ford to do is to replace Ford parts, so long as any non-Ford parts we've installed haven't caused their failure. So would the header swap have led to, say, premature catalytic converter failure? Again, determining causation is tricky business.
Finally, and this applies to unmodified cars as well, sometimes a dealer will make a last-ditch attempt to deny your warranty coverage based on maintenance issues: "Failure to perform scheduled maintenance as specified in the Service Guide will invalidate warranty coverage on parts affected by the lack of maintenance . . .
"Ford Motor Company recommends that you retain all receipts covering maintenance on your vehicle, but Ford cannot deny warranty coverage solely for the lack of receipts or for your failure to ensure the performance of all scheduled maintenance." Here again, the use of "affected by" is causation language, and the final sentence about the lack of service records not being fatal is critical. After all, it's highly unlikely that you have a service receipt from the last time you changed your oil in your driveway.