This letter communicates that you do not agree with a company's policy on your warranty--that it's expired, does not cover a certain part, etc.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
<time of contact> I contacted you about warranty work on my <product>. <time of reply> I received word from <company name> that <warranty problem>. This is completely unacceptable.
<reasons for honoring warranty>
I have been a good customer of <company name>, and I believe I deserve better service. I would like <your preferred action>. I think this is a very reasonable request. Please contact me and let me know what you will do. Keep in mind, your actions will demonstrate very clearly in what regard you hold my future business.
encl: <List of enclosed items goes here>