The dash (Instrument Cluster) diagnostic mode will only give very little information regarding the ECU.
I.E. the only things you can deduce from the instrument cluster are
Coolant temp, (From sender)
Oil pressure, (From Sender)
Gear Position, (From the Gear lever position sender),
Speed. (From Transmission Sender)
The ECU gets its values from these senders, (and many more).
If these values are corrupted, then the ECU can be sent into limp home mode, (depending on the problem).
The best way to properly trace a problem is to do an ECU test.
But if any of the above (especially the Gear position) are stuffed, then at least you have some idea where a problem may lie.
Hope this helps