Weak supply side voltage is the #1 reason for FICM failure. The official Ford spec indicates that during cranking operations, voltage is not to drop to 9.3 volts for more than 10 seconds. Letting batteries dip below 10.0 volts shouldn't ever be tolerated in actuality. Fully charged batteries left sitting overnight should measure 12.7 volts or maybe a bit more. If you are seeing numbers below 12.5, it's time to get those batteries on a charger! We recommend a good desulphating charger like is offered in the Battery Tender and BatteryMINDER line of products, though any smart charger will do.
In addition to checking the batteries here is a list of grounds to inspect and clean up.
Grounds to be concerned with (in order of importance) include:
1. Common battery terminals
2. Ground connection from passenger battery at lower passenger side of the engine.
3. Ground frame to passenger battery (close to connection above and part of the cable).
4. Ground drivers battery to frame under the air filter. (Install additional cable between this point and driver’s side to be doubly protected).
5. Ground body attachment above drivers headlight.
6. Ground body attachment on driver side firewall.
7. Ground body attachment on passenger side firewall above the heater.
8. Ground body attachment on the passenger cable pigtail by the battery.
9. Frame rail to the body under the passenger side footwell.