To check the temperature sensor you can use the digital multimeter tool and check the resistance of the sensor.
A simple steps which allow to check the temperature sensor:
- Turn the system off (electricity) – safety us the most important
- Open the controller box
- Check if the connection is not loose
- If loose fix the connection by pushing the sensor wire inside the terminal
- Close the controller box and check if that fixed the issue
- Check the sensor wire resistance with multimeter (turn the multimeter switch to the resistance measurement) – reading should be around 1.101 kOhm at room temperature and around 1.250 kOhm for hot water
- If readings are different sensor might need replacement
This short advice should help you to solve the problem.