Boiler displays E21, may be linked to incorrect sensor readings or inconsistent operation.

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

NTC sensors not matching Atlantic specification, Incorrect replacement parts, Wiring cross-connection between sensors, PCB calibration error

Safety: Turn off power before inspecting sensors or wiring.

Verify parts: If sensors were recently replaced, confirm they are genuine Atlantic parts with correct resistance curves.
Check wiring order: Ensure flow, return, and DHW sensors are connected to the correct terminals as per the wiring diagram.
Reset to factory settings: If the boiler menu allows, a technician can reset sensor calibration parameters.
Professional diagnosis: Persistent E21 requires PCB and sensor verification by a service engineer.

Hard 4/5

NTC Sensor Set

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