Safety: Turn off power at the breaker and wait a few minutes before working on the unit.

Locate coil sensor: Remove the front panel and filters. The evaporator sensor is usually clipped directly to the aluminum fins or copper tubing of the indoor coil.
Check mounting: Ensure the sensor is firmly attached to the coil. If it has slipped out, re-clip it in the original position to ensure accurate temperature reading.
Inspect wiring: Follow the sensor cable back to the PCB. Look for pinched, cut, or abraded insulation. Repair or replace damaged wiring.
Test sensor: Disconnect the sensor and measure resistance at room temperature. Compare to Viessmann specifications. Replace the sensor if resistance is open, shorted, or far out of range.
Reconnect and test: Reassemble, restore power, and run the unit in cooling mode to confirm the A2 code is cleared.

Persistent A2 with a new sensor suggests a defective indoor PCB that should be replaced by a professional.

Indoor unit stops cooling; A2 error on display

Possible Causes