If the Avalon A1166Pro kernel log shows Fan3  Fan4  (as shown in the figure below), it means that two fans of the Avalon miner are faulty. Again, you can refer to the methods in this article for troubleshooting.
1. The fan cable is loose or damaged; its recommended to reconnect the fan interface or replace the fan data cable with a new one;
2. The fan is damaged, replace the damaged fan directly.