Hugh surprise but Fiats S/S isn't working right, I'm fairly sure Fiat use the same system across all models with it fitted.

Goes like this, 2013 Punto bought last year had the original battery and S/S never worked. A few weeks ago I bought a new AGM battery for it (same Ah as original but supposedly better than the EFB factory fitted).

After I fitted the battery I left the S/S sensor left unplugged overnight to allow a reset. Morning comes, plug it in, drive about a mile and worked perfect, came to a stop and the engine stops. It will do this for an entire journey stopping and starting every single time. However if I then park up, come back to car and set off again S/S doesn't work no matter the distance driven or if I have all accessories switched off. If I unplug and replug the sensor in before setting off it immediately works and once again will S/S every time I apply the handbrake and put it in neutral.

Clearly the S/S system thinks the battery is up to snuff, I queued in a drive-thru recently and it start/stopped 4 times in a row, only rolling about 5ft forward at a time so battery is definitely tip-top. It just seems to forget this information unless I replug the sensor each time
Any ideas?

Better answered in the 500 section

Ive not had mine work in 3+ years..

But Im not missing it

It may be the stop-start sensor on the battery is duff? They do sell them on eBay but I can’t remember for how much, if the stop start has not worked for a long time then an old battery may have damaged the sensor or the sensor was the problem in the first place made worse by the old battery, now brought to light by the new working battery?

Well I decided to leave it to it’s own devices for a few days and stop bothering with unplugging the sensor. S/S hasn’t cut in once but then this morning start the car EML on and stop/start unavailable message. I drove to work anyway didn’t have time to faff about, when I got there I unplugged the sensor and restarted the car, EML went out but S/S error remained given sensor was still unplugged.
Plugged it back in before I drove home and once again S/S worked faultlessly no errors or lights. So I’m going to guess it’s the sensor... shame they’re bloody expensive!