There would be a stored code/s in the PCM if the sensors were faulty or the readings were out of the norm. If the PCM commands the light on, it will store a code!

Losing the buss signal from the PCM to the cluster will make the light come on, but not store a code in the PCM. By checking for codes in the cluster control module, you can tell if it lost communication with the PCM. Most of the gauge readings and lamp commands come through the PCM first, then to the cluster over the buss wires. The gauge self test will show you that the gauges and lights work, not if you are getting communications or if the cluster has any stored codes.