E05 displayed and thermostat cannot control heating; relay output remains off.

Safety first: This involves mains voltage. Isolate power at the main breaker and verify with a tester before working on wiring.

• Step 1 – Confirm demand: Check that the thermostat is actually calling for heat (set temperature above room temperature, mode set to Heat). Verify that the call‑for‑heat indicator appears on the display.
• Step 2 – Check wiring: Remove the thermostat from the wall base and verify that live in (L), neutral (N) if used, and switched live output to the boiler or heater are correctly connected to the Atlantic backplate terminals. Tighten any loose screws.
• Step 3 – Measure output (advanced): With power restored and extreme caution, measure voltage between the heating output terminal and neutral when there is a heat demand. If there is no output voltage and E05 is present, the internal relay/triac is likely faulty.
• Step 4 – Inspect for overload signs: With power off again, inspect the PCB for burnt tracks or a cracked relay/triac. If the connected load exceeds the thermostat’s rating, plan to use an external contactor in the replacement installation.
• Step 5 – Replace thermostat or relay module: On most Atlantic smart thermostats, the output device is not field‑replaceable separately. Replace the thermostat base or full unit with an identical Atlantic model rated for the load.
• Step 6 – Test operation: After replacement, restore power, re‑pair with the Atlantic app if needed, and test heating operation through several on/off cycles.