Hello, I am John, a Ford dealer technician.
I hope I can help solve your problem quickly and accruately.
The easy way to check is to take the yellow and white wire that goes to the fuel pump and ground it out. Get someone to cycle the key. If the guage goes all the way to full with it grounded then you need to replace the sender, if not replace the guage and slosh module as a pair.
I hope this answer has been helpful to you! If you have any further questions please feel free to ask.
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