Australian Car Repair Issues? Ask an Auto Mechanic for Answers ASAP
What make/model and engine?
Have you done a compression test on all cylinders with all spark plugs out?
Do you have injector pulse on the cylinder 3 injector plug?
Your fault could be the ECU, but I need to ask these questions first to see where you are.
Have you got a noid light, for testing injector pulse?
Sounds like the injector circuit is on at all times, instead of pulsing.
The injector control circuit could be grounded within the harness, or the ECM is faulty.
If you have proved the engine, and all other components are ok, and the engine runs on 3 with the injector disconnected, AND a noid light stays lit instead of flashing, then yes the fault is internal of the ECM.
I would get an exchange ECM.
Repairs are out of the scope of the average home mechanic.
I hope I have been of assistance to you today.
All the best to you.