Boiler displays E13; flue gas temperature too high or sensor fault.

Safety: Turn off power and gas. Combustion and flue work must be carried out by a qualified technician.

• Check flue terminal: Ensure the CTC flue terminal is not obstructed and that flue sections are correctly assembled and supported.
• Inspect flue gas sensor: Locate the flue gas temperature sensor on the flue hood or outlet. Check that it is firmly fixed and that the wiring is intact.
• Test sensor (technician): Measure resistance of the flue gas sensor and compare with CTC specifications. Replace if out of range.
• Check for overheating causes (technician): Inspect the primary heat exchanger for scaling or blockage and clean/replace as necessary. Verify burner pressure and combustion settings with a flue gas analyser and adjust according to CTC data.