if you disconnect the temp switch on top of the thermostat housing it causes the fans to kick in and the guage goes off so you probably have a faulty temp switch, but mine on my st170 was also triggering the eml light on the dash and giving code d073 when cycling through the diagnostic things on the dash, or p0118 on code reader, hope this helps mate, also how was your car running when the guage was off? my old escort si had similar problems and was holding back around 2,000rpm and at 4,00rpm, but was fine on the odd occasion the guage worked, either way get a code reader to accurately diagnose it as i was a muppet and tried to fix the escort myself and cost a fortune in new plugs, leads, coil pack and i even smashed out the ceramic from the cat thinking that was blocked and all for a £9 switch, good luck mate and let us know how you get on, cheers, roy