The route runs through farmland and woodland with some residences, crossing into Mahoning Township. PA 551 crosses Norfolk Southern's Youngstown Line and becomes Erie Street, heading into the residential community of Edinburg. Here, the route turns west onto Jackson Street and passes more homes before heading through farm fields and curving northwest. PA 551 comes to an intersection with US 224 and turns northeast to join that route on West State Street. The road heads through wooded areas and crosses the Mahoning River and CSX's New Castle Subdivision railroad line. PA 551 splits from US 224 by heading north on North Edinburg Road, continuing through wooded areas with some farmland and homes. The road crosses US 422 and continues through more rural areas, heading north-northeast and entering Pulaski Township. The route becomes an unnamed road and runs north through more farmland and woodland with some residences, curving northwest and north again. PA 551 comes to an intersection with PA 208 and turns northeast to join that route, heading through woodland to the west of the Shenango River. The two routes turn east to cross the river and head into the residential community of Pulaski. Here, PA 551 splits from PA 208 by turning northeast onto the Pulaski Township-maintained Mercer Street, curving north as it passes between industrial areas to the west and I-376 to the east.