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

Safety first: Turn off power to the boiler and thermostat at the fused spur or breaker before touching any wiring. Verify with a non-contact voltage tester.

Steps to check:

• 1. Power-cycle system: Turn off boiler and thermostat power for 2–3 minutes, then restore power and check if E01 clears.
• 2. Inspect communication wiring: At the boiler, locate the Baxi low-voltage control terminals (e.g. OpenTherm/EBUS or room thermostat terminals). Ensure the two-core cable to the thermostat receiver is firmly connected, not loose or corroded.
• 3. Check polarity and continuity: If using OpenTherm/EBUS, confirm the two wires are on the correct terminals at both boiler and receiver. Use a multimeter (power off) to check continuity of each conductor from boiler to receiver.
• 4. Check receiver module: Confirm the Baxi smart thermostat receiver has power (LEDs lit) and is correctly seated on its backplate. Reseat the module if necessary.
• 5. Test with a simple link: If safe and you are competent, temporarily bridge the boiler’s room thermostat terminals (as per Baxi boiler manual) to see if the boiler responds. If the boiler fires but E01 persists on the thermostat, the fault is in the thermostat/receiver side.

When to call a professional: If wiring is intact and the boiler responds to a manual link but not to the smart thermostat, the communication module or boiler interface may be faulty and should be tested/replaced by a qualified heating engineer.