Most likely your Display unit is fine and the sensor is bad. But rather than make assumptions, it's a simple test to determine where the fault lies.

Start by clipping a 3,000 ohm resistor across the leads in place of the sensor. If the display is working properly you will get a readout around 77 degrees F (25C), thus confirming that the sensor is bad. (Remember that the ignition must be on.) If the readout is still blank using the resistor, check the power and ground connections to the display unit.

If the sensor is bad it can be easily replaced for less than $5. The Display unit is a different story.

Good luck, Barrie