When it is poisoned by excesses of harmful reagents and when it is starving for nutrients essential to its function.
An example: when is an oxygen sensor not a good oxygen sensor?
When oxygen is plentiful and the oxygen sensor “thinks” it is not because of either of the above problems.
Is a critical deficiency for the HIF oxygen sensor vitamin C? Can iron deficiency, alpha-ketoglutarate deficiency, and succinate excesses also be problematic to the function of this sensor? We know von Hippel Lindau factor deficiency is a problem as far as turning over HIF when oxygen is plentiful, and no doubt the proline hydroxylase deficiency is as well.