Fault C4 — Communication fault with Open Therm extension

What does C4 mean?

The boiler is in control mode but detects a communication fault with the Open Therm extension. This extension allows the boiler to communicate with Open Therm compatible thermostats or controls.

• Loose or damaged connections to the Open Therm extension
• Faulty Open Therm extension module
• Incompatible or improperly configured Open Therm device

Click steps to track your progress.

1. 1

   Check all physical connections to the Open Therm extension module.

2. 2

   Ensure the wiring is secure and undamaged.

3. 3

   Verify the Open Therm device connected is compatible and correctly configured.

4. 4

   If connections are good, the Open Therm extension may be faulty and require replacement.