List,
This may sound daft, but the man pages don't appear to mention it. I have a box that sometime gives me a warning about fan RPM failure (or rather, the RPM is low, so sensorsd logs that it is outside the limits).
Thing is, sysctl doesn't show me a readout. E.g.
bash-3.00# sysctl hw.sensors.13 bash-3.00#
As it currently stands FAN sensors disappear when the fan fails or is spinning very very slowly. Sensorsd understands this and reports the failure.