C12
Bosch EasyControl Multi-Zone or Second Circuit Control Fault

Bosch EasyControl shows error C12 and cannot control a second heating circuit or zone as configured.

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Safety Warning

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

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Possible Causes

Incorrect zone configuration in EasyControl, Missing or miswired Bosch zone valve actuator or relay, Unsupported multi-zone configuration for this boiler, Faulty external zone control module

How to Fix / Troubleshooting

Safety first: Isolate mains power to boiler and any external zone valves or pumps before working on wiring.

  • 1. Verify zone setup: In the EasyControl installer menu and app, confirm that the second heating circuit is enabled only if the boiler and system hydraulics support it.
  • 2. Check external controls: Inspect any Bosch-branded or compatible zone valve actuators, pump relays, or wiring centers connected to the EasyControl. Ensure control wires are correctly landed and secure.
  • 3. Test manual operation: Manually operate the zone valve lever (if fitted) to confirm that the circuit heats when opened, indicating that the hydraulic side is functional.
  • 4. Simplify configuration: Temporarily disable the second circuit in EasyControl and run the system as a single zone to confirm that the base control works. Then re-enable and re-test the second circuit.

Professional help: Multi-zone systems can be complex; if C12 persists, a heating engineer should review the wiring diagram, hydraulic layout, and EasyControl configuration.

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Repair Difficulty

Hard 4/5

Required Part

External zone valve actuator
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