A drill crew was drilling ahead when a third party pressure sensor attached to the stand pipe stopped working. The sensor was made up to the standpipe with a size “602” hammer union rated at 6,000 psi working pressure. The crew removed the faulty sensor and replaced it. The hammer union on the new pressure sensor had a size “1502” wing nut rated to 15,000 psi. The “1502” female threads in the wing nut appeared completely made up to the “602” male threads coming off the standpipe.