I'm assuming that you mean that the starter works fine but the engine won't catch and keep running. You need to check for spark and for fuel pump operation, not at the same time. If spark is good, suspect fuel pressure. You should be able to hear the fuel pump run for a second at the uncapped gas filler pipe when your assistant turns the key to RUN. It should also run when you engage the starter. If you can hear it, then you know that the fuel pump runs. Next would be to check the fuel pressure using a test gauge, if you don't have one you may want to find a Nissan technician to help you.
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...here that can help give me some pointers to look for or even maybe a diagnosis of why the car won't start but the heater (sometimes) and the lights come on but no crank or noise from the key or out si...