Boiler display shows E4 and shuts down; may occur after running for a short period.

Safety first: Turn off power and allow the boiler to cool. System water can be very hot and under pressure.

• Step 1 – Check system pressure and flow: Verify that the system pressure gauge shows a normal cold pressure (typically 12–20 psi for many residential systems). Ensure all isolation valves on the boiler supply and return are fully open.
• Step 2 – Confirm circulator operation: Call for heat and listen/feel for the system circulator pump running. If it is silent or excessively hot to the touch, it may be seized or failed.
• Step 3 – Bleed air from system: Air-bound piping can stop flow and cause overheating. Bleed air from radiators or air vents as per system design. Ensure the automatic air vent near the boiler is open and not leaking.
• Step 4 – Inspect high limit sensor: Locate the high limit temperature sensor on the supply outlet or heat exchanger. Check wiring connections for looseness or corrosion. If the sensor is damaged or reading incorrectly (requires meter), it should be replaced.
• Step 5 – Reset and monitor: After correcting flow issues, reset the boiler and monitor temperature rise. If E4 recurs quickly, a professional should check for scale buildup or internal blockage in the heat exchanger.

Note: Repeated high-limit trips indicate a serious flow or control problem that can damage the boiler if ignored.