S-Max 2.0 Tdci 2016 water temperature reading fault

[Rhythm_Man]

My S-Max has developed an intermittent fault in the water temperature reading. The electronic 'needle' rises from a normal temp to max in maybe 2-3 secs, and an over-temp warning pops up. Sometimes the 'needle' fluctuates rapidly between normal and max, and sometimes the needle disappears entirely! Sometimes it goes into limp-home mode and then the problem goes away again and I can drive it normally once more. Generally occurs around 15 miles into a 21 mile commute. Feels like it might be a sensor fault rather than genuinely overheating, but the fact it has only occurred (so far) when it's warmed up fully doesn't sit well with that. Any suggestions please? Is it something I can check or easily replace parts myself? If I take it to my Ford dealer it will be £80 for a diagnostic check and then I'm guessing it'll be £200 for parts and labour and VAT! 

[TomsFocus]

First check is the coolant level if not already done.  If the coolant is low, there's likely to be air in the system giving the sensor false readings.

Next check would be the sensor itself, however it comes as part of the thermostat assembly on these, so not cheap or easy to replace.  The sensor is not available separately from Ford, though there may be aftermarket suppliers offering them.

Genuine Peugeot Citroen Ford 2.0 BlueHDi Diesel Thermostat & Housing 9804160380 | eBay