This morning, I realised that the air conditioner was just recirculating air, rather than cooling it.
In fact, the temperature never went down after running the house fan first thing in the morning.
It was also using less power than usual... usually it's ~4.5Kw just for the AC when it's running.
Something's afoot. I checked everything I could, and it seemed to be working, so I could only assume it's something I couldn't check, like the compressor, or the refrigerant.