No doubt one of the experts will be able to give instructions on cleaning et
al.   I can tell you that it is a very common fault across the whole Peugeot
range.   I can also tell you that it does not mean the airbag is likely to
go off, quite the reverse I believe as it is less likely to work.   I also
seem to remember that many people have soldered the wires rather than use
what I believe is an inferior quality connector, but hopefully one of the
experts on the group (and there are a number to whom I for one am deeply
indebted) will comment.

Same problem on our 306.  There's a connector attached to the underside
of the seats which connect to the seat belt pre-tensioners.  Happens to
us every few months or so.  Solution is to just disconnect and reconnect
the connector.  I always do it with the ignition off, but never worry
much about how long I have it turned off for - often just pull over at
the side of the road, remove key, fiddle with connector and restart car
and drive off.

I'm sure someone will say that's not recommended and you should
disconnect the battery for 30 minutes before fiddling with airbag stuff,
but - so far - I've not had a problem doing it my way.  ;)

D