if an "A" is displayed on the dash this indicates a "Head Capacitor" Failure.
The Head Capacitor ensures that a steady voltage is maintained to the positive headsink and that large voltage spikes are filtered out. The logics monitors this at P2-2.
Performance of the truck will be reduced when this error is active.

Check the head capacitor for any abnormalities. swelling or bad connections or improper readings during testing.
Also make sure the battery is in good condition , a low battery can cause high amp draw and might cause strange problems.

There may be other things that might cause this issue but i'd start with these things first.