Seicento emissions problem

[Newtownjoe]

Hi guys,
My name is Joe and my daughter has a seicento 1999 with about
45,000 miles on it.Low mileage for its age.Its due for an NCT test
soon and she was told that the emissions are off the scale.She went
to a garage and was told an emissions filter was needed.She went to
get one in a Fiat garage and was told it was not the filter and to find
the problem would cost her in the region of 400 euro.She only paid 650
for it and she is shocked that it will cost this to find the problem let
alone to fix it.Any idia`s what she can do and any help would be
appreciated. Thanks Joe.