Getting the old one out might be a problem, they tend to be
frozen in place. If, as it sounds, you are off the road, then get
the old one out and take it with you so you can compare
it with the replacement, screw thread and number/colour
of leads. Nothing worse than the old is red, blue, yellow,
the new, red, green, white conundrum. You can buy cheaper
generic replacements, whether you want to risk this is
a personal decision. An internet search on Lamba Sensors
should get you guide prices, and AFAIR testers for
them don't cost an arm and a leg.  

The lambda sensor is in the exhaust down pipe before the cat and exhaust
system.  Replacing the exhaust is most unlikely to damage the cat unless the
mechanic was a total monkey.

Before going down the sensor route I would get it tested.  Any MOT
station/dealer can do this for you (at cost) but there is a possible
alternative depending on your local circumstances.

Check you local council to see if they have a vehicle testing station.  Most
do.  All they do is serivice council vehicles, test taxis for MOTs and meter
accuracy and do MOTs for the likes of you and me.  No discounted prices but
also no axe to grind or look for work becuase they DO NOT do
rectification/repairs on any vehicles other than their own.

They are also in my experiance very tolerant and understanding of  the
publics concern about keeping their cars legal and will always try to help
in areas where specialist kit is required for MOT purposes.  e.g brake
testing and emissions testing.

So, contact your local depot and ask them if they could run and emissions
test when they have a spare moment.  I'm sure they will but you will have to
fit in with them (not them with you!!!!!!!!!!!).  The emssions test only
take about 5 minutes and the Lambda sensor is in effect tested.

Nick /////

Do you know if all Fiats have these sensors. I have a similar problem on an
89' Ducato motorhome