I assume you've checked the radiator level. If it's too low the sensor will not work. If it's right on the edge it can cause your problem. Next, do you own a multimeter? If so you can measure the resistance (Ω) of the sensor. Warm up the engine and when it's working shut the engine off and use the test leads and put one on the wire going to the sensor after removing the wire and the other on bare metal on the engine. You should get a reading, write it down. When it doesn't work do the same test. If you get a similar reading the sensor is fine. If not it's most likely bad. If the sensor is good measure from the wire on the sensor to the negative terminal on the batter (assuming it's a negative ground system). If that reads fine find install the wire back on the sensor and remove the cluster gauge. Find the wire coming from the sensor and measure it again after disconnecting the gauges and measure the sensor side again. If it reads fine plug everything back in and start the tractor. If it works now you might of just had a bad connection. If it doesn't most likely it's your gauge cluster.