Let's assume that there is an "oil quality" sensor. Such a sensor might have an output range of 2V to 7V. With 2V being good quality and 7V being the worst. If the computer reads the sensor and finds a value of 10V, then you get a "Engine Oil Quality Sensor Circuit High" Likewise if the computer read 0.5V, then it might output "Engine Oil Quality Sensor Circuit low".
I would avoid taking it to a shop. They will just take your money and may or may no fix the problem. Instead, purchase a factory shop manual. Factory shop manuals are sold by the manufacturer. Avoid the manuals found at the auto parts store. A factory shop manual will have very detailed specific information and will cover the various sensors. Buying factory manuals on-line is the way to go. You can even find them on ebay.