Tempreture Sensor gone again only replaced 400 miles ago

[f1basgh]

I have a 2001 VW Polo Estate 1600cc engine code AKL , Having loads of trouble with fault codes being logged into ECU. One of which is coolant temperature sensor i changed this only 400 miles ago and the same faults are coming up on my scanner . It had an original VW black plastic coloured one fitted but on going to GFS they gave me a green one as the black ones they had were completely different . Do you think it is the correct part , according to VW my problem is the Estate 2000-2002 is made up of parts from the models below , even the Vin number is a Seat one . So when i go for parts its a nightmare . Even VW Dealer cannot find out the problem with immobiliser faults IE Engine Start blocked due to Immobiliser & in Immobiliser section Key Low signal and i have replaced , Immobiliser , coil pick up , had them all coded in including both keys but still getting same faults unreal .

The Face Lift VW Estate from 2000-2002 is made up from parts of the vehicles below
SEAT Ibiza Mk2
SEAT Cordoba Mk1
SEAT Inca
VW Caddy
VW Polo Classic not hatch back