Q: I purchased Tefillin for my son at his bar-mitzvah. He had them checked five years later when he was in Yeshiva, and the parshas were found to be pasul. Am I entitled to new parshas from the sofer that wrote them?
A: This depends of on nature of the p’sul. If the defect was clearly there from the time that they were purchased – e.g. a missing or improperly written letter – the sofer owes you new parshas even if this was discovered many years later (C. M. 232:3).
However, if a letter was cracked or faded, the sofer does not owe you new parshas. The reason for this is that it is possible that the parshas were kosher when he sold them to you, and the ink faded or cracked over time.
Even if a letter was cracked in a manner that may have been there from the time the Tefillin were first bought, the sofer would still be exempt out of doubt. Since the defect was discovered in your possession, we assume that it developed here later on (C. M. 224:1 and Kessef Hakodoshim 232:11).