A1
Viessmann
Indoor Room Temperature Sensor (NTC) Fault
Indoor unit runs briefly then stops; A1 displayed on indoor unit
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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
Open-circuit room temperature sensor, Shorted room temperature sensor, Corroded or loose sensor connector on indoor PCB, Faulty indoor main control PCB
How to Fix / Troubleshooting
Safety: Isolate power at the breaker before opening the indoor unit.
- Access sensor: Remove the front cover and air filters. The room temperature sensor (NTC) is usually clipped near the indoor air intake or on the evaporator coil frame.
- Visual inspection: Check the sensor lead wires for cuts or pulled-out connectors. Ensure the sensor is firmly clipped in place and not dangling.
- Connector check: Unplug and re-seat the sensor connector on the indoor PCB. Look for corrosion or bent pins.
- Resistance test: With power off, disconnect the sensor and measure resistance with a multimeter. At ~25°C, most Viessmann NTCs will read around 5–15 kΩ (check service manual for exact value). Infinite or near-zero ohms indicates a bad sensor.
- Replace sensor: If readings are out of spec, replace the room temperature sensor with a Viessmann-compatible NTC probe and reconnect.
If a known-good sensor still triggers A1, the indoor PCB likely needs replacement.
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Repair Difficulty
Medium
3/5
Required Part
Indoor Room Temperature Sensor (NTC)
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