The wiring is shown to be simple - with power coming from a junction that powers the injectors and only one connection, no splices or interconnections; the other four wires are shown to connect directly to the ECU/ECM.
I wish I believed the wiring diagrams were accurate. If they are accurate, how hard can it be? Just know that corrosion can set in between the connector pins and the wire's conductors themselves; this is impossible to 'clean', new connector pins must be installed.

If the car is high miles and sits outside a lot, this MIGHT be the cause, but I gotta tell ya, such issues seem rare in our cars.