The thing about the ECU is everything
upstream of it has to function before itíll join the party.
Safety switches, relays, sensors, ignition coils, starter, and even the wiring has to be on the same page before the Control Unit will talk to them.
Itís the last thing on the list of things mentioned of fails in the service manual when trouble shooting.
If you donít have a service manual, youíre missing out on all the good stuff.