i have a 05/f550 that will not crank with key
Is your security system activated? If you place your key in the ignition and turn it to the on position does the red light (usually) on your dashboard still blink? I've had issues with GM's doing this, something to do with the tumblers in the ignition, or if you have done maintance without disconnecting your battery. If the light is blinking still with keys your security system USUALLY disables fuel supply and sometimes the ability to turn/crank starter. This also comes from damaged programmed keys, your vehicle is not recognizing the key. If this is it, try googling around there is normally a procedure for resetting your security system. Simply disconnecting the battery does not fix this issue either.
Jump out the starter relay and see if it cranks, or remove the key cylinder and use a screwdriver and see if it cranks, see YouTube on how to use a screwdriver, and how to remove the key cylinder
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