How to Audit the Client Quality System - Product Failure caused by no Maintenance

[QAMTY]

Hi everyone

I have a question , your feedback is welcomed.

I´m receiving complaints of my client regarding a failure of a product. It seems the problem happened because of maintenance lacking.
When we delivered the products, this was accompanied by a manual where was explained the schedule of maintenance.

Now,I have to analize the problem, but it is not easy to do it because I can´t be sure how maintenance was performed.

How can we track the maintenance activities of a client in respect to the acceptance of claims?

Thanks