Display shows A16; water heater indicates sensor short-circuit or overrange on one of the NTC inputs.

Safety first: Turn off power before working on any sensor wiring.

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

• Step 1 – Identify sensor:
  • Use the alarm details or service menu to see which sensor input is affected (e.g. tank, flow, return).
• Step 2 – Inspect cable:
  • Follow the cable from the control board to the sensor, looking for crushed or cut insulation.
  • Check junction boxes for moisture.
• Step 3 – Disconnect and test:
  • Disconnect the sensor at the board and measure resistance.
  • If the reading is near 0 Ω, replace the sensor.
• Step 4 – Reset alarm:
  • Reconnect or replace the sensor, restore power, and clear the alarm.

Persistent short readings with the sensor disconnected indicate a fault on the control board input.