Diagnosing a temperature fault on a Synergie 2002.

[ElgarL]

I have a Citroen Synergie 2.0L 16v MPV (2002), and I LOVE it, but its developed a fault of not starting.

I was driving it fine when the STOP light and over temperature warning came on. However, the temperature gauge showed the normal temperature but the car did stutter a little. The lights went out and all has been fine for a week or so.
Yesterday I was out when the STOP light came on and the engine cut out and would not restart. Engine temperature was fine. I disconnected the green temperature sender wire and the car started up (fans ran all the time with it disconnected). Even this is not the fault though, as on the way home the engine cut out again (Had the AA van following me home). It started up just fine and continued the rest of the journey. Engine wasn't overheating.
Today car will not start no matter if the temperature sender is connected or not.

Additional symptoms.

Turning on the ignition will cause the temperature gauge to max out and go back to zero three times (does this once in a normal startup). When it does the three cycles it will not start.
No ECU codes have been thrown.

There is a blue wire for a separate temperature sender (shows the actual temperature on the gauge) but I've been unable to locate the actual sensor as the wire runs behind the manifold and can't be viewed.
I have ordered a new green sender but I don't know what type the blue one is (or where it is located).

[ElgarL]

For anyone interested I believe I found the fault.

It was not a temperature sensor fault, although one did eventually get thrown by the ECU. It was water ingress over a long period of time into the Fuel pump relay and its connector. Over time corrosion and water sitting in the connector caused all sorts of shorts and weird electrical issue, like a random relay clicking like crazy now and then.
Eventually the module blew up (cracked its case open) which led me to the fault. It blew a track on the internal PCB, which I repaired. It took a LOT of cleaning to get rid of all the corrosion in the plug, as replacing the loom would probably be a nightmare.
The car is now running like a dream. Better than it has in the 15 years I've owned it.