AEM Coolant Temp Gauge problem....

The gauge has been reading incorrect so put it down to a faulty sensor. A replacement is around £44 with a plug, fuck that!

So ordered a new 1/8" NPT sensor and plug from EFI Parts http://www.efi-parts.co.uk/index.php?productID=125

Cut and soldered new plug to existing loom and it still read incorrect.

Purchased a new gauge and loom from a friend, cut and soldered the new loom and sensor plug and it still reads incorrect.

I've switched the gauge over to rule that out but again, still reads the same

Any ideas?