There could be several reasons why the IC board for the 3S Caldina is not turning on or functioning when you press the Aircon AC button. Here are some possible causes for this issue:

1. **Blown Fuse**: Check the fuse box for any blown fuses related to the IC board or the air conditioning system. A blown fuse can prevent power from reaching the IC board, causing it not to turn on.

2. **Faulty IC Board**: The IC board itself may be malfunctioning and needs to be replaced. Sometimes, electronic components on the board can fail over time, especially in older vehicles.

3. **Faulty Wiring**: There could be an issue with the wiring connecting the IC board to the car's electrical system. If there is a loose connection or a short circuit, the IC board may not receive power properly.

4. **AC Compressor Issue**: If the AC compressor is not working correctly, it can prevent the air conditioning system from turning on, which could be associated with the IC board issue.

In order to diagnose and fix the problem, you may need to consult a mechanic or auto electrician who can perform a thorough inspection and testing of the IC board, wiring, and other components related to the air conditioning system in your 3S Caldina.