Thermostat shows E16 and indicates pump or boiler control error; heating system does not start.

Safety first: Boiler and pump circuits involve mains voltage. Isolate power to the boiler and wiring centre before opening any covers.

Steps to diagnose and fix:

• 1. Check boiler demand wiring: Verify that the thermostat or wiring centre "call for heat" terminals are correctly wired to the boiler's control input as per both Danfoss and boiler manuals.
• 2. Inspect relay contacts: In the wiring centre, locate the boiler/pump relay. Look for signs of overheating or burning. If the relay is modular, it may be replaceable; otherwise, the wiring centre PCB must be replaced.
• 3. Test continuity: With power OFF, measure continuity across the boiler demand contacts when the thermostat is calling for heat (you may need to simulate this per Danfoss instructions). No change indicates a faulty relay.
• 4. Check boiler safety circuits: Ensure that boiler pressure, overheat, and external interlocks are all satisfied. A broken safety loop can prevent boiler start and be interpreted as an output fault.
• 5. Replace faulty module: If the Danfoss wiring centre or relay is defective, replace with an identical model. Reconnect wiring exactly as before and test operation.

When to call a professional: Boiler control wiring should be handled by a qualified heating engineer or electrician.