Coolant temp sender. Cupra 160tdi

[Scotty33]

Having a bit of a saga with coolant temperature, the gauge reads somewhat less than the 90 degrees I think it should. Had the same issue before with passat 130's, cured by fitting new thermostat, so ordered one along with a temp sensor, to fit at the same time. Temp sensor I ordered was the older style green 4 pin. On trying to fit it found my BPX engine uses the grey 2 pin type, doh! Ordered another temp sensor which I fitted today. The car seems to run better without the previous black smoke, and the mfa is showing more reasonable mpg achievable, so job done there, but the gauge still reads low when hot, so where does it get it's feed?