A sensing apparatus for detecting fluid quality change using signals obtained from a fluid quality sensor for exposure to a fluid and a reference sensor positioned in a reference sensing means having a fluid trap section holding a part of the fluid. Sensing values obtained from the fluid quality sensor and the reference sensor are compared in determining an aging factor. An application of the sensing apparatus is an apparatus for detecting engine oil quality change in an engine system. With the sensing apparatus, a first aging factor can be calculated after an engine cold soak and a second one can be obtained during engine operation. With these two aging factors, in addition to oil quality change, failures in the engine system and sensors, and oil change events can also be detected. Additionally, since reference sensing values are provided by the reference sensor, power disconnections have least effects.