Thermostat shows C4 and cannot control the mixing valve for a mixed heating circuit; flow temperature is incorrect.

Safety first: Turn off power to the heating controller before working on the mixing valve wiring. Valve and pipes may be hot.

• Check actuator wiring: Verify that the actuator’s open and close wires are connected to the correct outputs on the controller and that neutral/ground are properly connected if required.
• Observe valve movement: With power restored and a heat demand present, watch or feel the actuator to see if it moves when the controller calls for temperature changes.
• Manually move valve (if possible): Some actuators have a manual override. Gently move the valve to ensure it is not seized.
• Verify configuration: In installer settings, ensure the correct actuator type and travel time are set so the controller can position it accurately.
• Replace actuator or valve: If the actuator does not respond or the valve is seized, a technician should replace the mixing valve actuator or the valve body.