Modern double-hung windows (the upper and lower window sashes slide up and down past each other) don’t use pulleys and sash weights to support the sash (the moving part of the window). Instead they have liners on each side that contain a spring assembly. If your sash won’t stay in the raised position, chances are that some part of the jamb liner hardware is broken. You might be able to spot a broken cord or other sign of problem by looking closely as you open and close the window. If you determine that the jamb liner mechanism is broken, you can fix it by replacing both jamb liners.
The first step is to find a source for the new jamb liners. The original manufacturer is the best source of replacement parts. If you don’t know what brand your window is and can’t find a label, search online (try “window parts”) for a window repair parts specialist that can help you identify the jamb liner and send you new ones. But you may have to remove the jamb liner first and send in a sliver of it. Another option is to scan or photocopy the end profile of the jamb liner and e-mail the image to the parts supplier. Temporarily reinstall the old jamb liner and sash to secure the house while you’re waiting for the new parts to arrive.
The photos show how to remove a common type of jamb liner on a Marvin window. Photo 3 shows where to find the information you’ll need to order a new Marvin jamb liner. Start by tilting out the sash (Photo 1). Then remove the jamb liner (Photo 2).