Furnace displays E04; typically related to low water pressure or water circulation issues on hydronic models.

Safety first: Turn off power before topping up water. Avoid over‑pressurizing.

• Check pressure gauge: If below the minimum mark, use the filling loop to raise pressure to the recommended cold value.
• Bleed air: Bleed radiators and high points in the system, then re‑top pressure.
• Inspect for leaks: Look for visible water around the Sime furnace and radiators. Persistent pressure loss indicates a leak that must be repaired.
• If pressure is correct but E04 remains: The pressure switch or sensor may be faulty and should be tested and replaced by a technician.