Boiler displays E04, pressure gauge at zero or very low, boiler will not fire

Safety first: Ensure the boiler is cool before topping up pressure. Do not exceed the maximum pressure indicated in the Sime manual (usually around 2.0 bar cold).

• Check pressure gauge: Confirm the gauge is below 1.0 bar. If so, locate the filling loop (usually a braided hose with two valves) beneath the boiler.
• Top up the system: Open one valve on the filling loop, then slowly open the second until you hear water entering. Watch the gauge and close both valves when it reaches 1.2–1.5 bar. Remove the filling loop if it is detachable.
• Reset the boiler: Press the reset button or power cycle the boiler. If E04 clears but returns within days, you likely have a leak or expansion issue.
• Inspect for visible leaks: Check all radiators, valves, and pipe joints, especially around the boiler and underfloor manifolds. Repair any leaks found.
• Check pressure relief valve discharge: Look at the copper PRV discharge pipe outside. If it drips when the boiler heats, the expansion vessel may be flat or the PRV may be passing.
• Engineer checks: A technician should test the system pressure sensor, recharge or replace the internal expansion vessel, and replace a faulty PRV if necessary.