Error 07; unit may start then quickly shut down, or may not start at all.

Safety: Isolate power before working on sensors or wiring.

Steps:

• Locate return NTC: Find the sensor on the return pipe to the heat exchanger. It is usually clipped or inserted into a pocket.
• Check wiring: Inspect the cable for cuts, burns, or crushed sections. Ensure the connector at the PCB is fully seated.
• Clean and reseat: Remove the sensor, wipe off any corrosion or debris, and refit securely to ensure good thermal contact.
• Measure resistance: If you have a multimeter, measure the NTC resistance and compare to Biasi's chart. Replace the sensor if readings are clearly wrong (e.g., near 0 Ω or infinite).

Tip: NTC sensors are relatively inexpensive; if in doubt, replacement is often more practical than extended testing.