On mine it will not start if one of or both of the batteries are not good.
I had issues 2 years ago with it not starting. Both batteries were less than 24 months old. One was bad and took the other one out with it.
I wish I could get the original quality Motorcraft batteries that came in it. They lasted over 6 years. The replacements from Advance, which is the top battery they sell last just over 24 months and they quit.
I am coming up on 2 years on this pair of batteries and expect it to give no start issues again before the year is up.