Service contracts are written around the car as it left the factory or the dealer lot. Most owners do not think about that until they add a part, change a setting, or let a shop make a modification they do not fully understand. The contract may say nothing about the change, or it may say a great deal, and the second case is the one that surprises people.
Find the modification wording
Search the contract for words such as modified, aftermarket, altered, performance, non-original, or equipment not supplied by the manufacturer. Read the sentences around them. Some contracts require you to report changes, some only allow a claim if the change did not contribute to the failure, and some state that a modified vehicle may be treated differently. Your job is to find out which kind of clause you have before the change happens, not after.
Ask the questions that matter
Ask the provider whether a change must be reported, how it should be reported, and whether the report must come before the change or after. Ask whether a change made by a shop, a dealer, or a friend is treated the same as a change you made yourself. Ask whether a change can be reversed, and whether a reversal must be documented. If the answer is unclear, get it in writing from the provider, not only from the seller.
Also ask how the provider decides whether a modification played a part in a failure. Find out who makes that call, what evidence they request, and whether you can see their reasoning. Those answers will tell you more about the contract than the coverage list does.
Keep the records that would help you
Keep the receipts for every part, the installation invoice, and a note of the date and mileage when the change was made. Store the records with the contract and the maintenance file. If you ever need to show that a part was installed correctly, those records are the proof you will want. A shop that will not write down what it installed is a shop you should think twice about, whether or not a service contract is involved.
Check before you change the car
If you are still shopping and expect to modify the car, put that question at the top of your list when you talk to each provider. A provider's answer to this one question can affect whether your contract fits your plans for the car. Use the ranking page on this site to see the companies covered here, but read each company's own contract before you assume it treats modifications the way you hope.