B

External cut off switch not used in UK: Ensure nothing is plugged into red I3 connection.
1. Connection plug for external switch contact is not plugged into PCB.
2. There is no feedback voltage at the external safety contact at the condensate lifting pump blocks the heat generator.
3. Switching point for the external temperature monitor is set too low.
4. Connection cable to the external temperature monitor is damaged.
5. External temperature monitor is defective.