Thermostat shows E03 and cannot connect to boiler via OpenTherm

Safety first: OpenTherm is low-voltage but is often routed near mains terminals. Isolate power to the boiler before touching any wiring.

• Check control mode: In the thermostat settings, confirm that the control mode is set to OpenTherm only if your Vokera boiler supports it. If the boiler is on/off only, change the thermostat mode to relay/on-off.
• Inspect wiring at boiler: With power off, open the boiler's electrical connection area. Locate the OpenTherm (OT+ / OT−) terminals. Ensure the two-core cable from the thermostat is connected correctly and not mixed with 230V terminals.
• Inspect wiring at thermostat: Remove the thermostat from its wall plate and verify that the same two-core cable is connected to the designated OpenTherm terminals.
• Check for damage: Look for crushed cable, signs of overheating, or insulation damage along the cable route.
• Test in on/off mode: If E03 persists, temporarily configure the thermostat for simple on/off control (if supported) and wire it to the boiler's room stat terminals. If heating works in this mode, the OpenTherm interface on either the boiler or thermostat may be faulty and require replacement.