Display shows A17; water heater indicates sensor open-circuit or underrange on one of the NTC inputs.

Safety first: Isolate power before handling sensor connections.

This alarm indicates that one of the NTC sensors is reading an unrealistically high resistance (open-circuit).

• Step 1 – Identify sensor:
  • Check the alarm details to see which sensor is affected.
• Step 2 – Check connections:
  • Ensure the sensor wires are firmly clamped in the terminal block or connector.
  • Look for broken conductors at strain relief points.
• Step 3 – Continuity test:
  • Disconnect the sensor and measure resistance; infinite resistance indicates a broken sensor.
• Step 4 – Replace if needed:
  • Install a new Nibe-compatible sensor and route the cable to avoid sharp bends.
  • Restore power and clear the alarm.

If the alarm persists with a new sensor, the control board input may be faulty.