s300 and SManager software questions & answers
Good troubleshooting on your part. The problem is likely to be the injector output transistors, which can be damaged if the injector wiring is ever shorted to 12V or if low impedance injectors are used. Once that happens they have a weak output that doesn't drive the injector very well.
IF you have narrowed it down to the ECU it is probably best just to discard that ECU as you could spend hours and lots of money trying to troubleshoot the old ECU and never resolve the issue. Replace the ECU and move on.