Boiler shows E17, often related to external control or thermostat issues; boiler may not respond to room thermostat.

Safety first: Turn off power before altering any control wiring.

User-level checks:

• Check that the room thermostat or programmer has power and is set to call for heat.
• Replace batteries in wireless thermostats if applicable.

Technical checks (qualified personnel):

• Verify wiring at the boiler control terminals matches the Chaffoteaux wiring diagram.
• Temporarily link the boiler's room thermostat terminals (if allowed) to confirm the boiler responds without the external control.
• Check continuity of the control cable and correct polarity for OpenTherm/eBUS connections.
• Replace the faulty thermostat or interface module if communication cannot be restored.

After correction, test full operation with the external control in both heating on and off states.