Corolla 1997 - Error 31

[Hans.T]

Heya, i have an Toyota Corolla 1997, there was a small frontal crash but nothing serious and airbags did not deploy the car been little driven since the crash and no issues what so ever but recently i had to change some body parts that were in the way of MOT and Battery was charged to full suddently after ignition Airbag light appeared and stays on even in ACC with engine off

I did check with Toyota dealer and error code was 31

Tho during the check they noticed that alternator power surges from 14v up to 16v in idle randomly and as a wild guess may have burned the the airbag's control unit or least damaged it.

How high the chance would be that it have taken damage for high current and is there anyway to verify?

My theory so far is because Battery was always nearly empty the alternators jumping did not manage to create power spike thanks to Battery draining that extra power, but as i got charged to full and then started it may have created power surge of 16V and damaged the ACU unit, getting quite clueless on what else possibly could cause the issue with airbag unit regarding power levels.

as the unit swap & alternator by Toyota is slightly too high gamble :S

Anyone else had anything similar or any ideas on how to proceed on to narrow down the issue at least a bit more :S