Does the coolant temperature sensor (the 4-terminal one mounted in the pipework at the back of the head) have a known historical problem with failure?

They have been superceded by VAG from the original Black colour to the present Green. A web search indicates they are a popular source of problems. mine has always periodically shown a coolant sensor circuit fault code, although without illuminating the engine fault lamp. In the main I haven't noticed any problems with the way the car drives, although recently I've noticed on occasions it is difficult to start when part warm, overfuelling massively until it is running..

I've changed it, so will wait and see if these issues are fixed

Does anyone know the functions of the two sensors incorporated in the housing? One is connected by pins A&B, the second by pins C&D - I'm guessing one half will be for the engine management and the other for the dash gauge??

Let us know if initial fuel economy changes

Does the coolant temperature sensor (the 4-terminal one mounted in the pipework at the back of the head) have a known historical problem with failure?

They have been superceded by VAG from the original Black colour to the present Green. A web search indicates they are a popular source of problems.

Yes - as you have just found out for your self. When I had the cambelt and tensioner changed not so long ago, I also had the dual temperature sensor, water pump and turbo pressure pipe changed for the later modified versions.

i have no complaints about performance or fuel consumption, just the occasional warm start difficulties..

70,000 miles & on its3rd sensor so I guess the answer is yes

Funnily enough, I just had the coolant temperature sensor replaced on my 1.8T (at 64k miles). Prior to replacing, the car suffered from occasional starting problems and the odd 'hiccup' under light acceleration. So far it's back to behaving itself (hope this is tempting fate).

I changed mine recently, rough/spluttery running was sorted

Anyone know what the failure mode actually is?

I've watched the live data from mine whilst running, and it showed the temperature as it should be for 99.9% of the time, but just occasionally I saw it read -48deg.C which i'm assuming may well be the failure, becoming intermittently open-circuit..? The engine fault light has never come on through all of this.