Air Bag light - Can't communicate with Airbag ECU

[chrismwall]

Hi all. Another thread to add to the millions but I can't figure this out.
2002 Renault Clio 1.5 Diesel
Story: airbag light randomly came on. Comes on when ignition lights, switches off, then comes back on in a few seconds

Tried so far:

• Checked the fuse
• Checked the airbag ecu is getting power
• Checked wiring from ecu plug to underseats etc - all connects.
• Plugs under the seat - sockets were removed and wired together with a connector as had a problem before. They look fine and not changed.
• Changed Airbag ecu with one from another car - still no change
• Diagnostics - plugged in and it will detect everything else and fault codes but will not communicate with airbags/airbag ecu to get a fault code

So I'm completely lost. Any suggestions as to how to diagnose the problem, turn off the airbag lights (and pass an MOT), etc
Thanks in advance