Could be many different things... Check the 10 pin connectors (black and white-salt/pepper) between the firewall and the upper intake. Make sure there is good contact between the male/female pins. Pull them apart and gently spread the male pins so they make good contact. It is free and may/may not help your problems.
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General Stonewall Jackson has been quoted saying that when the time of war comes,
“My advice is to draw the sword and throw away the scabbard".