Boiler shuts down with E04; temperature rises rapidly before lockout.

Safety: Turn off power and allow the boiler to cool fully. Do not bypass safety thermostats.

• Check system pressure: Confirm the pressure gauge on the CTC boiler shows the correct cold pressure (typically 1.0–1.5 bar for sealed systems). Repressurise via the filling loop if low, following the user manual.
• Bleed radiators and pump: Air in the system can cause local overheating. Bleed all radiators and any automatic air vents. On some CTC models, the pump has a bleed screw—carefully vent air with a cloth ready for small water discharge.
• Verify valves are open: Ensure the boiler flow and return isolation valves beneath the unit are fully open.
• Check pump operation (technician): With a heat demand, feel the pump body for vibration and warmth. If the pump is not running, check voltage at the pump connector. Replace the circulation pump if seized or electrically faulty.
• Inspect primary heat exchanger (technician): If E04 recurs, the primary heat exchanger may be scaled or blocked. Isolate, drain, and chemically flush or replace the CTC primary heat exchanger as per service instructions.