Boiler displays E04 and will not start; may occur after power interruption or wiring work.

Safety: Isolate power to the boiler before accessing internal components.

• Power cycle: Turn the boiler off at the fused spur for 30 seconds, then turn back on and see if E04 clears.
• Check connections: Remove the front cover (qualified person) and locate the flow temperature sensor on the flow pipe leaving the primary heat exchanger. Ensure the sensor plug is firmly connected and the cable is not damaged or pinched.
• Inspect sensor: Look for signs of corrosion, water ingress, or physical damage on the flow NTC. If the sensor body is cracked or wet, it should be replaced.
• Resistance check (qualified person): With power off and wires disconnected, measure the resistance of the flow NTC with a multimeter. Compare to Alpha's temperature/resistance chart. A reading of open circuit or near zero ohms indicates a faulty sensor.

If wiring and sensor are sound, the main control PCB may be at fault and should be tested/replaced by an Alpha-approved engineer.