Boiler shows E17, no response to room thermostat or OpenTherm controller, heating does not start.

Safety: Isolate power before accessing the control terminals.

• Check control settings: On the Defro control panel, verify that the correct control type (on/off thermostat, OpenTherm, or internal control) is selected according to the installation manual.
• Inspect wiring: Open the terminal cover and check that the thermostat or controller wires are firmly connected to the correct low-voltage terminals.
• Bridge test (on/off thermostats only): With power off, temporarily bridge the thermostat terminals at the boiler using a short wire. Restore power; if the boiler now responds and heats, the external thermostat or its wiring is faulty.
• Controller check: If using a Defro modulating controller, try resetting it or testing with a known-good controller.