Do you have digital or regular gauges? If you can get an idea of your actual water temperature and measure the resistance of the sensor the values are 104 degrees F - 1300 ohms, 203 degrees F about 220 ohms. Resistance gets less the warmer the sensor gets. It sounds like you have a sensor problem. Have you cleaned the terminals and connectors in the circuit? You can also ground the wire to the sensor and your gauge will read hot to check the gauge.
1994 Blazer Tahoe LT, 4X4, 4 Door, Loaded, Lots Done to Restore, Still More to Go
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