Thermostat shows F4 and cannot communicate with the Protherm boiler; heating does not start on demand.

Safety: Turn off power to the boiler and thermostat before touching any wiring. eBUS is low voltage but is connected to mains-powered equipment.

• Step 1 – Check wiring polarity and connections:
  • Remove the thermostat from the wall plate.
  • Verify that the two eBUS wires are connected to the correct terminals (e.g. + / - or eBUS A / B) on both thermostat and boiler.
  • Tighten all terminal screws.
• Step 2 – Inspect cable:
  • Follow the cable route between thermostat and boiler where accessible.
  • Look for crushed, cut, or spliced sections.
  • Repair or replace damaged cable with a twisted pair cable recommended by Protherm.
• Step 3 – Reduce interference:
  • Ensure the eBUS cable is not run parallel and close to mains power cables over long distances.
  • If necessary, reroute or shield the cable.
• Step 4 – Reset boiler and thermostat:
  • Turn off power to both devices for 2–3 minutes.
  • Restore power to the boiler first, wait until it is fully booted, then power the thermostat.
  • Check if F4 clears.
• Step 5 – Test with short cable (diagnostic):
  • Temporarily connect the thermostat to the boiler with a short, known-good cable.
  • If communication works, the original cable run is faulty.
  • If F4 persists, the eBUS interface on the thermostat or boiler PCB may be defective and require professional replacement.