I know this sounds like some other posts on here but I have tried the solutions suggested by them and nothing seems to work. I had previously used this MacBook Air with Ubuntu 15.04 a couple of years ago and I got the wifi working quite easily but I am having real problems this time.
The wifi card is detected and recognised but will not find any SSIDs. I am currently using a USB wifi card but this MacBook Air only has 1 USB port so this is a pain! At the moment the wifi menu shows the Broadcom BCM4321 as disconnected. I have installed the firmware-b43-installer and checked to see if it is hard or soft locked (it is not). It is shown in the alternative driver section as using the Broadcom 802.11 Linux STA wireless driver source from bcmwl-kernel-source(proprietary). Wifi worked fine on this MacBook both in Mac OSX Snow Leopard and in Windows 10. I do have a copy of the Boot Camp drivers if these are any use...
Following advice in another similar question, I have disabled the alternative driver. Currently iwconfig gives:
$ iwconfig wlan0 IEEE 802.11 ESSID:off/any Mode:Managed Access Point: Not-Associated Tx-Power=20 dBm Retry short limit:7 RTS thr:off Fragment thr:off Power Management:off
rfkill does not show that anything is blocked.
$ sudo lshw -c network *-network description: Network controller product: BCM4321 802.11a/b/g/n vendor: Broadcom Limited physical id: 0 bus info: pci@0000:02:00.0 version: 05 width: 64 bits clock: 33MHz capabilities: pm msi pciexpress bus_master cap_list configuration: driver=b43-pci-bridge latency=0 resources: irq:16 memory:90300000-90303fff *-network description: Wireless interface physical id: 2 logical name: wlan0 serial: 00:1f:5b:e2:f5:a2 capabilities: ethernet physical wireless configuration: broadcast=yes driver=b43 driverversion=4.13.0-32-generic firmware=666.2 link=no multicast=yes wireless=IEEE 802.11 02:00.0 Network controller : Broadcom Limited BCM4321 802.11a/b/g/n [14e4:4328] (rev 05) dpkg-query: package 'bcmwl-kernel-source' is not installed and no information is available
I uninstalled bcmwl-kernel-source as recommended in the instructions.