Temperature Gauge

Has a scare on the motorway, the temperature gauge suddenly went fully over to the right, the LED flashed a couple of times then stopped. Having pulled off the motorway and lifting the bonnet, I could find no problems. The fan was kicking in and then stopping as normal. Just to be safe, I shorted the thermo switch so the fan remained on for the rest of my journey. The temperature gauge did not return to normal.

In the morning, I switch the ignition on and the gauge goes to a bit over 3/4 and I hadn't started the engine.

Is the problem the temperature sensor or something else?

Might just be a poor connection, clean up the wire on the sensor if you can. Had similar situation with my oil temp, occasionally goes up to 146deg , if I wiggled the wire it returned to normal, cleaned up and not problem since

Checked all the leads but the problem remains.
It is odd that it happened the way it did.

Check it's not earthing out if the insulation on the sender wire is stripped somewhere.
I had a very similar problem years ago with the sender wire earthing where it was touching the rocker cover because of dodgy insulation (albeit on a mk1 Astra rather than a Golf!)

Yes, that was my first thought but the wires are good.

the wire must be shorted soemwhere between the sensor andf the fusebox, its the red/black one to the black sensor on one of the coolant flanges.

if its a GTI theres a coolant level warning relay 42 or 43 above the fusebox which is also tapped into the coolant temp wire, pull it out and check it for damage plus the wirign to the relay socket

At last I believe I have found the problem, it is the relay. Having removed the relay all looked in order, I decided, however,to open the relay and on doing so I found that there was some gunge inside clearly from one of the components which had failed and the grunge was shorting the circuit. Obviously, I won't know for sure until I fit a new relay, but I'm confident that replacing the relay will solve the problem.