The useful life of an engine oil may vary considerably; therefore, an on-board sensor would be helpful in monitoring the oil's condition continuously. In this work, in situ macro and micro oil-condition sensors have been developed and tested in two sets of engine-oil samples. It is found that after suppressing the high output current associated with fresh engine oil, the microsensor's output correlates with the total acid number (TAN) of engine oil. Because an oil's TAN will increase with use, this microsensor has the potential to detect the end of useful oil life and signal the need for an oil change.