The tester primarily reacts to the increase in acidity of oil. As the oil degrades, it becomes more acidic and will begin to pit metal engine partsThe red LED #10 reading usually means that the level of acid in the oilhas reached a maximum acceptable level, and the oil should be changed.owever,other contaminants( such as dirt, soot, water, antifreeze andmetal particles )also influence the reading Water or antifreeze in the oimay indicate a bad seal or head gasket Metal particles result from unusual wear of bearings, pistons, rings, etc. Excessive carbon or sootmay be caused by poor compression and excessive blow-by Fuel leaking can dilute the oil and may cause an artificially good oil quality reading. If test results show that oil quality does not drop as expected
over time, or seems to improve, check engine for fuel leaks. Try a dropof oil on paper. Look for a gray ring. )If a significantly worse oil rating than expected is displayed, we recommend having the engine checkedor send a sample to an oil laboratory for analysis

TROUBLE SHOOTING

#10 LED lights constantly after the tester is tumed on.
The button cells are low. Replace the button cells with two new button cells of the same type ( 3V button cell, CR2032 or equivalent. 

#7-8-9 LEDs flash in turn
The sensor is dirty, or you added oil on the sensor before turning on the power switch.

My old oil tests like new.
The sensor was not cleaned well before the test, or your oil may not be
as bad as you think. Some oil can last over 10000 miles.

My test results are inconsistent.
Make sure that the surface of each oil sample under test reaches the
required height as indicated in Figure 2. Sensor must be well cleaned
after each test.