Cellular pathways are ordinarily diagnosed with pathway inhibitors, related gene regulation, or fluorescent protein markers. They are also suggested to be diagnosed with pathway activation modulation of photobiomodulation (PBM) in this paper. A PBM on a biosystem function depends on whether the biosystem is in its function-specific homeostasis (FSH).An FSH, a negative feedback response for the function to be performed perfectly, is maintained by its FSH-essential subfunctions and its FSH-non-essential subfunctions (FNSs). A function in its FSH or far from its FSH is called a normal or dysfunctional function. A direct PBM may self-adaptatively modulate a dysfunctional function until it is normal so that it can be used to discover the optimum pathways for an FSH to be established. An indirect PBM may self-adaptatively modulate a dysfunctional FNS of a normal function until the FNS is normal, and the normal function is then upgraded so that it can be used to discover the redundant pathways for a normal function to be upgraded.