Dtc # C1280

Hi, We got a 2017 Rav4 (8000 km)that came in a while ago with the 4wd system disengaged. Warning light on. So I started to diagnose the issue..ending by replacing the 4wd ecu. I cleared the ecu's memory but didn't do the ecm's memory then.

So client went for about near a month trouble free until it came back on again(C1280) this week.So I ran techstream on it again,did a health control,no freeze data on the DTC. I cleared all dtc's, cleared both 4wd and ecm's memory. Checked bus can, all ecu's communication seemed good.

My question here is: has anybody ran into this similar problem here? And if yes could you fix the issue, and how did you?

Diagnosis trouble chart says if the DTC is output, to diagnose ecm, and if its not output, to change 4wd ecu.
What do they mean by: is the DTC output or not?