F21 error, unit does not start, sometimes after sensor replacement.

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

Possible Causes

Incorrect type of NTC sensor fitted, sensor wiring connected to wrong PCB terminals, PCB expecting a different sensor set, damaged sensor harness.

Safety first: Isolate power.

Steps:

Verify part numbers: Check that all NTC sensors are genuine Saunier Duval parts with the correct specification for your model.
Check connections: Compare wiring to the official wiring diagram to ensure each sensor is plugged into the correct PCB socket.
Inspect harness: Look for splices or non-original connectors that may indicate previous incorrect repairs.
PCB configuration: If the board was replaced, ensure it is the correct version and properly configured for the appliance.

Warning: Do not mix sensors from different models. Incorrect readings can compromise safety.

Hard 4/5

NTC Sensor Set.

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