Furnace displays E5 and will not fire, often accompanied by low or zero system pressure

Safety first: Ensure the system is cool before topping up pressure to avoid scalding.

• Check the pressure gauge. If below the recommended level (often 1.0–1.5 bar cold), use the filling loop to slowly add water until the correct pressure is reached.
• Bleed air from radiators and then re-check pressure, topping up again if necessary.
• Inspect visible pipework, radiators, and the boiler casing for signs of leaks (staining, damp patches, or drips). Repair any leaks before repeated top-ups.
• If pressure drops repeatedly with no visible leaks, the internal expansion vessel may have lost charge. A technician can test and recharge or replace it.
• If the gauge shows correct pressure but E5 persists, the water pressure sensor or its wiring may be faulty and should be tested and replaced by a professional.