code E30
- ECCS C/U Error signal. No power supplied to the ECM or ECM detects an insufficient voltage supply.
- No or low or high supplied power to the ECM.

The most common causes :
- EGI relay 1 open or short circuit
- EGI Relay 2 open or short circuit
- Main C relay open or short circuit
- Ignition relay open or short circuit
- Open ground circuit
- Open across fuse: F6- EGI relay 1 or F7- EGI relay 2 open circuit
- Corroded or broken wires at the ECM or lower left side step plate relay box and body ground.
- Internal open or short in the ECM
- Abnormally low or high ECM to sensor output voltage detected

From my experience the most common issue i have seen causing this issue is either a bad ground on the frame side of the truck where the grounds connect to the stud on the side of the frame under the battery. Remove the cables and wires and make sure there is no paint on the surface where the wire connectors make connection, if there is scrape the paint away and then reconnect the wires.
Secondly mcfa had issues with the relays on these units and had to do a parts update on the relays, the old ones were black and were updated to a green colored one. Intermittant signal from these relays can cause the EGI circuit to trigger a code.
If you find it still has the black ones in it replace them. It mentions above checking the black relay box under the foot step on the left side, i have rarely had to deal with these relays so that would only be a step to take if you do not find any problem with the other relays.

Also check the fuses in the fuse box and make sure none are blown, since this code indicates an open circuit in the EGI circuit it could just be a blown fuse. (uncommon)