Circuit description (2003 Service Manual)
The powertrain control module uses the engine coolant temperture to monitor the engine for an over temperture condition. This condition occurs when the coolant temperture is above 132 degrees C (270 degrees F). When an over temperture condition is present, DTC P1258 will set. Blah, Blah, Blah
With it not having a code I would say it would be the gauge.
Another thought, wasn't there some problems with insturment panel clusters and the gauges acting up mentioned on here before ?
This one is about the speedo.
http://www.elcovaforums.com/forums/s...strument+panel
Same here speedo
http://www.elcovaforums.com/forums/s...strument+panel
Complete cluster not working
http://www.elcovaforums.com/forums/s...strument+panel
Not found any on the temp gauge though ?