Boiler display shows A21; unit may shut down unexpectedly during operation.

Safety: Turn OFF power to the boiler. Allow water temperature to drop before touching sensor wells.

Steps:

• 1. Locate the outlet sensor: Find the sensor on the outlet header or sensor well labeled as outlet or supply temperature.
• 2. Inspect wiring: Check the sensor harness for damage, loose plugs, or corrosion at the connector. Reseat the connector firmly.
• 3. Check sensor placement: Ensure the sensor probe is fully inserted into the well and secured. A partially inserted sensor can read incorrectly.
• 4. Compare readings: If the control has a diagnostic menu, compare the displayed outlet temperature to an external thermometer on the outlet pipe. Large discrepancies suggest a bad sensor.

If A21 remains after checking connections, the outlet sensor should be replaced. If a new sensor does not resolve the fault, the control board may be defective and requires professional service.