my mate has an au an it wont start turn the key an you can here the fuel pump but thats it checked all the relay's,fues's, got a new starter motor new computer an new battery.every now an then will start for like 15 times straight an runs fine then wont start at all for 40 times could it be something in the security system cutting it off not sure an an thats that only thing that we can think of if any ones had previous problem the same or knows anything about it input would be greatly appreciated. cheers

Try hitting the left side of he dash beside the steering wheel or in my case i hit the gear leaver towards me and some times bloody hard always does the trick
cheers
Alan

do it crank over

If it's an auto, it could be the inhibitor switch (white unit attached to passenger side of trans). This basically tells the computer systems which gear you're in. When it fails, it thinks you're in gear when you're not, so it refuses to start - and it can be intermittent.

Can be a failing BCM/BEM in the lower dash in front of driver's left knee. Sometimes a good thump there might get it going.

If you have an aftermarket alarm system (eg Mongoose), these can sometimes interfere with the Smartlock, and it will start intermittently. Mine was doing this, so I had to have the immobiliser function of my Mongoose system disconnected by an auto sparkie.

Very good chance it's the inhibitor switch. When it plays up like this, try cycling the shifter down through first and then back in park and turn the key again. Used to happen to me on the rare occasion. They aren't too expensive.