Mark, It seems to me that one complaint in five years is a reason to stay in business rather than abandon it, especially where the complaint comes three years after you did the work. All businesses get complaints; no repair shop would be in business if it operated under the standard you set for yourself.

Take John Allen, the Inde that DubMutant praises, as an example. According to Allen's website, he guarantees his work for one month or 1,000 miles, apparently whichever is shorter. See

J.F. Allen Mercedes Repair, Shelburne MA | Car Talk - Mechanics Files

You should reconsider whether you want to go out of business due to this single incident.