Technical Engine Code P0116 ECT Sensor Problem

[damo07]

Hi, I've been getting this error code P0116 on my Fiat Bravo 2009 T-Jet 1.4 150bhp for the past 3 months, temperature gauge drops to zero and doesn't move, radiator fan comes on immediately after turning ignition on and runs continuously until engine is switched off. My local garage has replaced the ECT sensor which worked until I went on a longer journey 30 minutes +, then the error came back on the dashboard. Took it to an auto electrician who then re-made some wiring connections (not exactly sure which ones) and again problem solved until 2 days later when on a longer journey 30 minutes + on motorway. Local garage is now suggesting adding an extra earth cable from the ECU. Has anyone had this same issue or can anyone help me understand what it is and how it might be fixed? Any help much appreciated. Thanks