Boiler displays E04 and will not start; may occur intermittently when hot water or heating is demanded.

Safety first: Ensure the boiler is cool before topping up pressure. Do not exceed the maximum pressure indicated on the boiler label.

User-level checks:

• Check the pressure gauge on the boiler. If below about 1.0 bar (cold), top up using the filling loop as described in the Chaffoteaux user manual.
• After filling to 1.0–1.5 bar, close the filling valves securely to prevent over-pressurisation.
• Reset the boiler and check if E04 clears.
• Inspect visible pipework and radiators for leaks (damp patches, green staining, or drips).

Technical checks (for qualified personnel):

• Verify correct operation of the water pressure sensor or pressure switch; check wiring and replace the sensor if readings are unstable or incorrect.
• Check the expansion vessel pre-charge and recharge or replace the vessel if the diaphragm has failed.
• Inspect the safety valve for passing water to the discharge pipe; replace if it does not seal properly.
• Locate and repair any hidden leaks in the heating circuit.

If pressure repeatedly drops after topping up, a leak or expansion vessel fault is likely and should be addressed by a heating engineer.