Thermostat shows E12 and fails to control one or more underfloor heating zones via the wiring centre.

Safety first: Wiring centres typically contain mains voltage. Isolate power at the main switch before opening the enclosure.

Steps to diagnose and fix:

• 1. Check bus wiring: Open the wiring centre and verify that the communication bus (often 2‑wire or 4‑wire low-voltage) is correctly connected and firmly clamped. Look for broken or loose conductors.
• 2. Verify polarity and terminals: Ensure that +/− or A/B terminals are not reversed between thermostat and wiring centre. Match the markings exactly as per Danfoss documentation.
• 3. Confirm addressing: If using multiple zone modules, check DIP switches or software settings for correct zone addresses. Duplicate addresses can cause E12 errors.
• 4. Inspect wiring centre PCB: Look for burn marks, corrosion, or water ingress. If present, the wiring centre PCB may need replacement.
• 5. Test with a short bus run: Temporarily connect the thermostat to the wiring centre with a short, known-good cable to rule out cable faults in the building.
• 6. Replace faulty module: If communication works with a test cable but not with the installed wiring, replace the damaged cable. If no communication is possible even with a test cable, replace the wiring centre or thermostat as indicated by further tests.

When to call a professional: Multi-zone wiring centre diagnostics should be handled by an experienced heating installer.