F12 displayed, unit stops, often associated with sensor wiring issues

Safety: Isolate power before testing sensor circuits.

• Identify which sensor: Use the service manual or diagnostic menu to determine whether the fault relates to the DHW sensor, primary flow sensor, or return sensor.
• Inspect harness: Follow the sensor cable from the sensor body to the PCB. Look for crushed insulation, rodent damage, or contact with hot surfaces.
• Check connections: Reseat the sensor connector at the PCB. Ensure no pins are bent or corroded.
• Measure resistance: Disconnect the sensor and measure its resistance at ambient temperature. Compare to Viessmann's NTC chart. Replace the sensor if readings are out of range.
• Reset and test: After repair, restore power, clear F12, and run the heater. If the error returns, the PCB input circuit may be damaged and require replacement.