1991 Nissan Pathfinder starter

Do you mean it cranks over and will not start or just won't crank at all/nothing/nada?

Cranks and w/n start

Get a helper disconnect a sparkplug wire or 2 and ground it to the engine at least 3/16 away from ground -have helper crank engine over-do you have a snapping blue spark? If so-you have a fuel related problem, check the fuel pressure to rule out the fuel filter/fuel pump/pressure regulator and listen to the injector/s are they pulsing or hook up a noid light. No snapping blue spark continue to troubleshoot the ignition system-power input to the coil/coil packs,coil's resistances,distributor pick-up coil, ignition control module, cam and crank sensors and computer Note: If it doesn't apply disregard it

No crank at all/nothing/nada:

Could be the starter, starter relay, park and neutral switch,Clutch switch and ignition switch assuming the battery and connections are good.