Airbag ECU code 31 - what does it really mean?

[ajuniper]

My mothers Yaris has just turned its airbag light on for no obvious reason. The car's not been involved in an accident.

Jumpering the connections on the diag connector gives code 31 which concurs with the Toyota diagnosis of wanting to replace the airbag ECU for a large sum of money.

What does this code 31 really mean? Is it normal on the Yaris for the airbag ECU to just fail like this, or is it more likely to indicate a wiring problem somewhere? If so, what connections need looking at, and which are safe to check with a voltmeter / ohmmeter?

Are there any instructions anywhere for extracting the airbag ECU?

With symptoms like this, is sending the unit away for a code reset likely to be a proper fix, or is the code likely just going to keep coming back?

The car is a July 2004 UK model French built Yaris with a 1SZFE engine.

thanks
Andy