How to Fix / Troubleshooting

Safety: Turn off power and allow the system to cool. Surfaces and water may be very hot.

Steps:

Check pump operation: Listen for the Buderus circulation pump when there is a call for heat. If silent or excessively hot, it may be seized.
Verify valves: Ensure all main isolation and zone valves that should be open are not accidentally closed.
Bleed air: Air pockets can stop flow. Bleed radiators and high points, then restore pressure.
Inspect strainers: If your system has Y-strainers or dirt separators near the furnace, clean them to restore flow.

If L3 persists, a technician should test the pump, check for internal blockages, and verify sensor operation.

L3 error, furnace cycles off on limit, radiators or ducts may be very hot

Possible Causes