1999 ford Explorer, does not start?

On my '97 (4.0 V6 OHV) it was a weak battery. It turned over but would not start.

By "new f/f" do you mean new fuel filter? I'd suspect something related to that. Air in the fuel line?

a couple of things come to mind:

No Fuel: Check or hear if your fuel pump is coming on when turning the key before starting it. If you don't hear it, check the fuel pump cut- off switch( I think its on the front passengers kick panel, not sure). Could also be a bad relay. Swap it out with the A/C relay. Also check to see if the fuel filter was put in the wrong way.

No ignition: You mentioned you got a tune-up. Check for loose ignition wires or coil wires.

Bad IAC valve:. Step on the gas while trying to start the engine to prevent stalling.

Bad crank sensor: Although rare, they do sometimes go bad and it will prevent your truck from starting.

With the PATS system, does the "theft" ligh

flash when the ignition is turned on? Just a thought