Thermostat shows E15 and the app indicates ‘Communication bus error’ between Ngenic control module and expansion modules (e.g., zone controllers).

Safety: Bus wiring is usually low-voltage but may run alongside mains. Turn off power to the control module before working on wiring.

• Inspect bus connections: Check that the + and − (or A/B) terminals on all Ngenic modules are wired consistently and firmly.
• Check cable integrity: Look for damage, crushed sections, or moisture ingress along the bus cable.
• Reduce bus length/devices: Temporarily disconnect distant or non-essential modules to see if E15 clears.
• Power-cycle system: Turn off all Ngenic modules, wait 30 seconds, then power back on to reinitialize the bus.

If E15 persists with minimal devices connected and correct wiring, the bus transceiver on a module (often the main Ngenic control module) may be defective and require replacement.