B1004 error from Alfa Romeo 159 Airbag ECU

[papr]

Hello, the Airbag warning symbol lights in the dashboard of my Alfa 159 (model 2008).
MES shows this error when reading the ECU:

"B1004 - connection to ECU grounding."
"ECU supply voltage is below or above specifications."

I cleared the error and the light went out - but lighted up again when I drove on a bumpy pavement.
So it looks like an unstable ground contact.
But - the Airbag ECU does not have an extra ground wire.

Also, MES says in the error explanation:
"The reason is the ECU did not receive signal from Sensor."

That sounds different - now what exactly did the ECU complain about:
a Difference between its _own_ voltage and the battery's one?
OR
b Failure of a sensor signal?

[shpuncik]

check battery and ground cable froms negative pole to body

[papr]

I overhauled the battery compartment recently, and cleaned and greased the ground connections on this occasion.

The error seems to hint to a) the ground connection of the ECU and b) sensor connection.
What is the relation between these two?

[shpuncik]

if you sure grounds are OK, check whether battery itself and alternator are OK. just try monitoring airbag ECU voltage in live data - plot graph while driving to check for any abnormalities. most likely there is drop or spike in voltage and airbag ECU is not happy about that.

[papr]

Ok, I'll try that on next occasion, and report again.