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If you have no or low fuel pressure then most likely the pump isn't running or running slowly.
Check fuses if you haven't already. You can also try swapping the fuel pump relay with another and see if that helps.
While cycling the key or cranking the engine check fuse 18 in the power distribution center and see if it has power. If it does then you know the relay is sending power to the pump, if the fuse isn't blown then power is at least leaving the fuse box to the pump. If you don't have power there then make sure the relay has two power feeds. It receives two power feeds and then the ECM supplies ground to turn the relay on.
If you do have power at the fuse then the next thing you'd want to do is check for power and ground at the pump when cranking or cycling the key. If there is power and ground at the pump but it won't run then the pump will need to be replaced.
If you find the pump does run but there is little or no pressure then either the pump is bad or the power or ground are there but poor.