Thermostat shows E9 and cannot drive the heating actuator.

Safety first: Turn off the power at the breaker. Do not work on live circuits.

Steps to troubleshoot E9 (output stage fault):

• 1. Disconnect the load:
  • Remove the thermostat from the wall.
  • Disconnect the heating load wires from the output terminals.
• 2. Test the thermostat without a load:
  • Restore power with the load disconnected.
  • Clear any errors and set a high temperature to call for heat.
  • If E9 clears, the fault may be in the load or wiring.
• 3. Inspect load wiring and device:
  • Check the underfloor heating mat, valve actuator, or radiator for shorts or visible damage.
  • Use a multimeter to measure the resistance of the load; compare with manufacturer specifications.
• 4. Reconnect or replace the load:
  • If the load tests good, reconnect it and test again.
  • If E9 returns immediately, the thermostat's output triac is likely damaged.
• 5. Replace the thermostat:
  • The output triac is PCB-mounted and not user-serviceable. Replace the thermostat unit.

When to call a professional: If you suspect a shorted heating mat or actuator, a heating specialist should test the insulation resistance and repair or replace the load safely.