Boiler shows A12, no response to thermostat or control signals

Safety: Turn off power before working on control wiring.

Steps:

• Check that the room controller or thermostat has power (batteries or mains).
• Inspect the low-voltage control cable between the IVT boiler and controller for damage or loose terminals.
• Ensure the polarity and terminals match the IVT wiring diagram (e.g., BUS+, BUS− or OT1, OT2).
• Reset both the boiler and controller according to their manuals.
• If communication is still lost, a technician should test with a known-good controller and replace the faulty device or PCB interface.