I am pretty sure that there isn't any CAN network in a 2001 Spider and also no stability system so I assume if the codes you are stating are correct have another meaning.
I think P1314 has something to do with catalyst/lambda
No idea about U1601

 

The 3.1 ECU has pin out A12 CAN-L line and A29 CAN-H line


U1601 is the one that's lighting the MIL and its the one I cannot clear.

 

I agree with Henrik_45. I looked on the Fiat Forum to see what was listed there; because there are rather more Fiats around there could be more examples (the generic code lists do not apply to FCA vehicles in all situations). There I found threads that indicated it was a ‘catalyst damaging misfire’. Presumably neat fuel getting to the catalytic converter is not a good thing!
Best to see if you can get your car connected to dedicated Alfa/FCA diagnostics.
Generic lists show it as being a throttle position sensor problem.

 

Interesting post here

DTC remove U1600 EDC15C7

1

mhhauto.com

I know my code is 1601 but my code 1601 also refuses to clear either.

I can clear P1314, then returns a moment later but it does not light the MIL whereas 1601 does same as 1600 same as above.

Coincidence?

 

Looking on the Fiat Forum at older models U1601 is related to a problem with the key and immobiliser. If you have another key try that. Also see if the aerial behind the ignition lock is not loose. Perhaps if it is moving out of place it might just trigger the immobiliser for a millisecond triggering a misfire.

 

I thought the same, so I removed the covers. Looks ok.

I then removed the yellow key code box and up came U1600 and key lamp stays on.
Refit the box, key lamp goes out and I can clear U1600 so I don't think its that.

Also remove the Meta 99t2 box but still U1601 and P1314 appear

Bridged the wires on the clutch switch but again no luck

I don't like the fact that the U1600 was impossible to clear and I have the same with U1601

 

Going back to the O2 sensors, I wonder if I post a result of a test from MES, can you interpret please?

I have absolutely no knowledge of the working of these devil things...and perhaps any other test results I could take.