This is what I bought based on the advice on the fiat forum for a nice member.
https://www.ebay.co.uk/ulk/itm/161715574443
This is a reader and all the cables you need.
You need this software
http://www.multiecuscan.net which needs a Windows PC
I had to register the software to work with the reader from eBay. It's €50 to register which was about £40.
It took no effort to use. You remove the plastic panel between the drivers footwell (just below the steering wheel) and the driver's door. The panel just pulls off. The OBD port is behind it at the top (I'd seen a fiat engineer do this in the past when they forgot to reset my service indicator).
You plug the reader directly in and then select on the software the module you're wanting to connect to. For some you use the yellow adaptor cable (which comes with that reader), the software prompts you when it needs it.
I'm sure you can get a reader that works without having to register the software but to be honest I'm not begrudging paying £40 when someone has gone to the trouble to write software to help us all out.
The software is easy to use and you can quickly see any errors.
I know others have probably alternative readers to suggest but this was the one I was pointed towards. I also bought another which I haven't even tried but it was only £5.99 so no too bothered if I don't have any use for it.
Hope that helps.