Not starting up just turning over - 2001 Jaguar XJ8

A year, make, model, and engine size would help but I will through this at you.

Instead of just trying to start the car turn the key to the run position and wait 3 seconds. Turn the key off and immediately turn the key back to the run position and wait another 3 seconds. Turn the key off then try to start. Do this every time you try to start the car. If you symptom never occurs when you start the vehicle like this than I would suggest taking a closer look at the fuel system.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

Won't start how ? Does the engine turn (crank) when the key is turned to the start position ? Not start an run but turn or spin ! A crank - no start Or doesn't turn or spin this is A no crank - no start . These two different conditions would need different diagnostic steps .

Please provide more information.

"I put the key in the ignition, turn it and the warning light goes out, I turn the key to start but there is nothing..." or

"I put the key in the ignition, turn it and the warning light goes out, I turn the key to start but there only a ticking/clicking sound..." or

"I put the key in the ignition, turn it and the warning light goes out, I turn the key to start but the engine only turns over and doesn't catch."

GM cars have bad problem with security system. If you key turn light and will start other time will stick and need shaking to turn over and won't start just replace the key. Security system need some times to refresh the memory.
1.turn key ON for 10min.
2.turn key OFF 10sec.
3.turn ON 10min.
4.turn OFF 10sec.
5.turn ON 10min.
turn OFF ON and start
total of 30 min. to refresh the memory
the next step is more expensive and need some skills.
on top of the ignition switch is a sensor go bad
replace ignition switch with the sensor and do the learning process from step 1 to 5

Have you checked the automatic shut down relay ?
It powers the injectors from a 20amp fuse number 5.

Sounds like the pump has seized.

Neutral Safety Switch ? located at the bottom of the steering column. Mine is tricky, have to pull the shift lever while turning over to get it started. happened after i replaced ignition switch & Neutral Safety Switch

Hello: Try this, Turn key on, try to start, let k/switch come back to run, let set for 10min(security light out) turn key off 10sec, turnkey back on let set 10min, turn key off 10 sec, turn key on let set 10min. turn key off 10sec. turn key on start car if it doesn't start and the security light is flashing you have a passlock problem and it will require a new lock cylinder assembly and the above learn procedure.

check the neutral safety switch sometimes when they are going out they will start in neutral