An email or letter of apology regarding defective goods without offering a refund, return or replacement of product. Politely point out that the product may have been misused by customer and the warranty period has expired. Offer a trade-in / repair instead.
Thank you for providing us with feedbacks on issues you have with our product. We are sorry that [PRODUCT] have not lived up to your expectation in the past [number] months of reliance performance. You are entirely justified in not being completely satisfied with [PRODUCT], and we are always eager to service any product that does not live up to standards.
We are very concerned about the issue you raised and have requested [our technician / the manufacturer] to inspect the malfunctioning <? echo $PRODUCT; ?>. The preliminary assessment is that it is caused by [number] problems. [It is apparently being used for a significantly higher volume than what it was built for (as indicated in both the sales brochure and the user's guide with which you were provided).] [In addition, there are indications that <? echo $PRODUCT; ?> has not been used properly [STATE how the product has been misused]. This can lead to [DESCRIBE how this results in the malfunction], increasing the wear and tear of a number of mechanical parts.] / [The manufacturer insisted that the goods are of sufficient quality there is nothing wrong with them.]