[CWizard]

CG- If it's a Smith's mechanical temperature gauge, there is no separate, replaceable sending unit. You have to replace the entire gauge assembly. You can get it repaired but the cost of a replacement is usually less than repair plus 2-way shipping. Check with Nisonger or Gauge Guys for details on either option.
Strange failure mode - they usually fail low. Maybe you have an electric Smith's?
Good luck.