Those plug in readers do work pretty good. A light came on in my Subaru and the guy plugged in the reader. He came and asked me about my shifter and I was having trouble getting it in gear. I was impressed it took him only about 2 minutes to find the problem. The big wheel-in computer analyzers don't seem as good. One time my engine was running so bad the car was shaking and black smoke was coming out of the exhaust. The guy comes out and says the computer says there is nothing wrong , LOL. Turned out to be the O2 sensor.