Boiler locks out with E10; may indicate high system pressure or pressure sensor error.

Safety: Hot water under pressure can cause burns. Allow the system to cool before relieving pressure.

• Check pressure reading: Compare the boiler's displayed pressure with a mechanical gauge if available. If pressure is above the recommended maximum (often ~25–30 psi), it must be reduced.
• Relieve pressure: Carefully open a drain valve or bleed water from a radiator into a bucket until pressure drops into the normal range.
• Inspect expansion tank: Tap the expansion tank; it should sound hollow at the top and solid at the bottom. If it is full of water or leaking, it may have failed.
• Check fill valve: Ensure the automatic fill valve is not stuck open. Close any manual feed valves.

If pressure is normal but E10 persists, the pressure sensor may be faulty and should be replaced. A technician should also verify expansion tank pre-charge and system sizing.