Display shows E04 and brewing stops; sometimes accompanied by unusual pump noise.

Safety first: Switch off and unplug the machine. Hot water and steam may be under pressure inside.

Step-by-step:

• 1. Remove and clean brew unit: Open the service door, remove the Siemens brew unit, and rinse it thoroughly under warm water. Pay special attention to the metal or plastic filter screen. Let it dry and reinstall correctly until it clicks.
• 2. Run rinse cycles: With the brew unit reinstalled, run several automatic rinse cycles or hot-water draws to flush the internal lines. Use a container under the spout to catch water.
• 3. Descale the machine: Perform a full descaling program with Siemens-approved descaler to clear scale from the pump, thermoblock, and valves.
• 4. Listen to the pump: During a rinse or hot-water cycle, listen for the ULKA pump. A loud buzzing with little or no water flow suggests a blockage; silence suggests a failed pump or electrical issue.
• 5. Advanced checks: If comfortable, remove the side panel to access the pump and overpressure valve. Inspect silicone hoses for kinks and check that the overpressure valve is not stuck with scale.
• 6. When to call service: If E04 persists after cleaning and descaling, the pump or overpressure valve may need replacement. This involves working with pressurized hot water circuits and should be handled by a Siemens technician.