temperature gauge problems Uk model 1992 K reg 100 2.3 quattro AAR

temperature gauge appeared to work when it feels like it but on test deflects fully if shorted and warning buzzer works for over temp and autocheck shows temp problem so replaced "large"4 pin sensor on top of pipe to right of block with new 3 pin from euro car parts £25 (Audi replacement now has 3 pins £75 !!) !! everything worked fine now two weeks later back to same situation gauge fails to work but tests out correct full deflection etc conned Audi dealer into checking their techie said sensor was ok don't want to keep spending £25 for 2 weeks of temp gauge working. there is another small temp I think sensor under the same pipe directly below the one I replaced does this have any affect on temperature gauge ? what does it do? any advice gratefully received