Thermostat display shows E01 and cannot communicate with the Immergas boiler; heating does not start

Safety first: Turn off power to the boiler and thermostat at the main switch or breaker before touching any wiring. Do not work on live circuits.

• Step 1 – Power cycle: Turn off the boiler and thermostat power for 2–3 minutes, then restore power. Check if E01 clears.
• Step 2 – Check BUS wiring: Remove the thermostat from its wall plate. At the thermostat and at the boiler, verify that the BUS terminals (usually marked BUS, OT, or +/- on Immergas boards) are firmly connected and not corroded or burnt.
• Step 3 – Verify polarity: If the BUS is polarity-sensitive on your Immergas model, ensure + from the boiler goes to + on the thermostat and – to –. Reverse connections can cause E01.
• Step 4 – Inspect cable: Follow the BUS cable route. Look for crushed insulation, rodent damage, or splices. Repair or replace damaged cable with equivalent 2‑core low-voltage cable.
• Step 5 – Test with short run: If possible, temporarily connect the thermostat to the boiler with a short test cable (1–2 m). If E01 disappears, the original cable run is faulty.
• Step 6 – Board/module check: If wiring is good and E01 persists, the thermostat communication module or the boiler main PCB BUS interface may be defective. At this point, contact an authorized Immergas service center.