This is just from threads that I've read here, so do your own search, but it seems that the sensor can also accumulate dirt which interferes with the signal, which is another free fix.

I'm glad that my car lacks that particular sensor, although it has its own trouble spots.

If your pressure sensor really is a switch, and I don't know that it is, you can check it with an ohm meter. Pressure switches should be closed (conducting) with the system off, assuming that the system has sufficient refriderant.