53 Pa.C.S. § 8424
(a) Minimum time periods for taxpayer response.—
(b) Requests for prior year returns.—
(2) A local taxing authority may make a subsequent request for a tax return or supporting information if, after the initial request, the local taxing authority determines that the taxpayer failed to file a tax return, underreported income or failed to pay a tax for one or more of the tax periods covered by the initial request.
This subsection shall not apply if the local taxing authority has sufficient information to indicate that the taxpayer failed to file a required return or pay an eligible tax which was due more than three years prior to the date of the notice.