Engine turns, tries to start, but shuts of emediately on 2002 Ford F-250 Super Duty

After engine warms up and running, not always, but has done it three times, u turn the key, engine turns tries to start but quits, no codes on scanner, i replaced the Idle Air Valve, but did it again. Fuel filter is 6 mo. old. Also having hard time locating the fuel pump relay.

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2 answers
THERES AN INERTIA SWITCH ON FORDS THAT SHUTS DOWN THE FUEL PUMP ALSO. IT ALSO SOUNDS LIKE A THEFT DETERRANT SYSTEM HAVE YOU CHANGED KEYS RECENTLY
sounds more like a fuel pump. have the pressure checked and i bet you will find little or no pressure.

Roy