Scanner from the loaner tool bin at the auto parts store is a high failure item. Just because they let people use it does not mean it works. It would be nice if it always worked, but it is likely to be abused. There are several data streams that come on different pins, it's possible -- likely -- that the data stream from your car does not work with the scan tool, but from other cars it works okay. You have to compare to a known good condition, then see if it also works with your car, or not. Just because the guy gives it to you does not mean it is reliable.

You can have your own scan tool for 30 bucks, give or take. Go on eBay or Amazon and get an ELM-327 module that works with your device -- phone/tablet. Then go on the app store for the device and buy an app. OBDFusion is a good one, TorquePro is another. There are free versions that you can use on a trial basis. Technically, you can use the free version for everything forever, but the free version has a limited feature set, the fee-based versions are the way to go.

You can read codes and reset the computer. The fee-based units allow you to collect data logs that you can feed into a spreadsheet and build a graph. The free versions do not support graphing.