My 2006 X5 3.0 diesel won't start. Sometimes it does and when it does I can hear the pump but can't when it doesn't. I got it plugged in and the computer said: rail pressure monitoring, rail pressure too low during start - intermittent. I know this might sound obvious to alot of you but I don't want to rush out and purchase a pump just in case its something else. All suggestions welcome and thanks in advance.