LEGOPARTY

If a shop says they can’t install a new muffler without first replacing the exhaust pipes—because they’re “rusted out”—be sure to check them yourself (or get a second opinion) before authorizing the additional repair. Even though this can be a legitimate excuse, it is often used dishonestly to sell expensive repairs.
How can you tell if a part is too rusted or not? A light coating of rust on the surface of exhaust pipes, mufflers, or converters is perfectly normal and will not prevent a good welded installation. A part doesn’t need to be replaced if it doesn’t leak, or if it only has light surface rust.
However, if a part is so rusted that its surface has bubbles or scales on it, or if the part can be punctured with a screwdriver (using normal hand pressure), then it probably needs replacing. In some parts of the country (by the ocean or where roads are salted in the winter), exhaust parts may rust quickly. In other areas, those same parts could easily last 100,000 miles or more.