On the touchscreen, if the nozzle temp shows -30°, please follow the below to do the troubleshooting.
1. The below cable circled in red is the thermistor. Make sure it is inserted into the small hole correctly and properly.
2. Replace a new hot end.
Here is a video about how to replace the hot end.
If the issue still exists, please kindly send an email to firstname.lastname@example.org and we will help you out. Thanks.