Hmm. Before you rewire stuff, maybe experiment a little.
I am pretty sure it is true, as others here have said, that if the car sees two keys at the same time, it won't start.
I did not know that the 556U is active at all times if the keysense wire is not hooked up. That is odd. The more I think about it, the more I don't believe it----if it were true, the little relay inside would be on all night, and your battery would get drained. DEI does some unusual things, but not like that.
Anyway, as I was saying, experiment first. You want to verify if all your keys are still programmed to the car or not.
1. Unplug the 556U to temporarily eliminate it from the system.
2. Try to start the car with each of the keys you have, one at a time-------do the work now? If so, it's possible that the 556U is on all the time as others have said.
3. I know the remote starter works, but if you're particularly ambitious, remove the key that's in the 556U and try that in the ignition too. And you might even take one of your other keys, put that in the 556U, and try it that way too.
If the keys STILL don't work, then something is wrong with them or they have lost programming somehow.
But since you say they work sometimes, I think this is more likely to be an install-related problem.
Remember, for Fords, you need TWO working keys to program more. So if you still have two good keys, it's easy to fix-----if you have only one good key, you'll need a dealer or locksmith.
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A little about that 556U. I have just used a few of them when we ran out of our regular product-----they seem useful and versatile, but overly complicated too.
--About the keysense wire: The unit has two keysense wires; a positive and a negative.
Your Ford has a positive keysense wire (BLACK/ pink at the keyswitch or steering column), as mo said.
If you choose to connect the keysense feature (it can't hurt), read the instructions with the 556U and be sure you are using the positive keysense input.
The idea here is that when you insert a key, it disables the 556U, so the car can't see two keys at once.
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How do you have your 556U wired? Please describe what you have done with each of the wires. This is how it is supposed to go:
1. Red: constant, positive, 12 volts power.
2. Black: chassis ground.
3. Blue: when this wire receives a ground, the 556U turns on. It should be connected to either the blue or blue/black from the Viper remote starter------one is called "status", the other is "ignition 2". Either one of those wires puts out a ground when the remote starter is on.
(Since I do this professionally and speed is important, I simply jump the black and blue together, and connect them BOTH to the ground-when-running output from the remote start----yes, it is strong enough to power up the 556U-----but it is probably just as well if you connect the black directly to ground.)
For most installations, this is it for the 556U----all of the other wires would be shortened, taped up, and not used.
I like to just rip the unused wires out of the harness entirely, so the job looks neat, but that would make it difficult to hook up the wires later if needed.