The cache is not at the posted coordinates.
The original DuPage County Courthouse was built in 1868 on land purchased from Warren L. Wheaton. A new DuPage County Courthouse replaced this structure in 1896 and housed many important events. Clarence Darrow defended George Munding in a trial in 1924 that made regional headlines. The first sentencing in Illinois of a convicted criminal to go to the electric chair occurred here in 1931. A jailhouse annex was constructed in 1959, although it was later demolished. The courthouse operated until 1990, when the present day DuPage County Courthouse was built. In danger of being demolished, the building was listed on the National Register of Historic Places in 1978. A lightning strike in 1988 destroyed the original tower, though it was subsequently rebuilt. National Louis University purchased the building in 1993 to operate as a satellite campus. In the mid-2000s, the building sold again and converted into condominiums.
Near the posted coordinates is a plaque that will have the numbers needed to solve.
Label those numbers as ABCD.
This cache can be found at:
N41 51. D B (C-A)
W88 06. A (B-D) (D-C)