Fault 03 Code 21 — Room temperature sensor faulty, short circuit

What does Fault 03 21 mean?

The Worcester Bosch Greenstar CDi Boiler displays Fault 03 with code 21, indicating a short circuit in the room temperature sensor built into the FW 100 or FR110 controller.

• Internal short circuit within the room temperature sensor
• Wiring shorted to another wire or component

Click steps to track your progress.

1. 1

   Visually inspect the wiring connected to the room temperature sensor for any signs of insulation damage or exposed wires.

2. 2

   Ensure no metallic objects are in contact with the sensor or its wiring.

3. 3

   If a short circuit is suspected or the fault persists, contact a qualified heating engineer to replace the faulty controller or sensor.