Thermostat shows E4 and will not operate heating; may be accompanied by blank or flickering temperature display.

Safety first: Isolate mains power at the consumer unit before touching any 230V wiring.

Steps to check:

• Verify supply voltage: With a multimeter and power restored, measure voltage at the thermostat L and N terminals (for mains-powered Salus models). It should be within the specified range (typically 220–240V AC). If low or unstable, investigate the supply circuit.
• Tighten terminals: Turn power off again and check that live and neutral conductors are fully inserted and clamped in the thermostat backplate. Loose connections can cause voltage drops and E4.
• Inspect for heat damage: Remove the thermostat head and inspect the backplate and PCB for signs of overheating, discoloration, or a burnt smell near the power components.
• Check external loads: Ensure that no external devices are incorrectly powered from the thermostat terminals (e.g. pumps, valves drawing more current than specified). Overloading can damage the internal power supply.

Resolution: If supply voltage is correct and wiring is sound but E4 persists, the internal power circuit is likely damaged. Replace the thermostat unit. Do not attempt component-level repair unless you are a qualified electronics technician.