Calcium deposits can coat the thermal sensors, insulating them and causing inaccurate readings.
If your machine is still under warranty, contact Krups support before attempting any internal repairs. If out of warranty, Error 09 is usually a "repairable" fault that requires a relatively inexpensive replacement part rather than a whole new machine.
Look for "scorched" marks or blown capacitors on the main PCB, which suggest a power surge damaged the temperature regulation circuit. How to Prevent Error 09
Prevention is significantly cheaper than repair. To keep your Krups machine running smoothly:
If the machine overheats, the thermal fuse may blow to prevent a fire, resulting in a total loss of temperature data. Step-by-Step Troubleshooting