2000 frontier wont fire,

If your service engine light is not on it is probably in the fuel delivery category. There was a recall on the fuel sending unit, corrosion in the connection. My truck, 2001 3.3 v6 4x2, was having this problem, found that I was losing voltage to the fuel pump where the wiring passed thru the fuel sending unit. We found that my fuel filter was pretty clogged also.

the fuel sending unit is just for the level the fuel pump is separate a problem with the sending unit would give you false level readings at the gauge. check plugs, wires, coils ect. check fuel pressure, even check if your mass airflow meter is dirty. how is your timing belt it could have broken.

The fuel pump and the sending unit are separate connections. The problem I had was that the wiring between the outside connection and the inner connection for the fuel pump had failed. Had to drop the fuel tank twice, suggest testing the actual pump before installing. The pump is connected to the bottom of a splash box inside the tank, connections are part of the sending unit assembly. My fuel gauge read fine. I have had to replace my timing belt, pull the distributor cap and see if the rotor turns when you crank the engine. If it does not move with the motor the belt is broken.