What year is the car? If it is newer than 96 than an OBD tester will tell you whats wrong with your car. I was putting in new headers and unplugged my o2 sensor and had started up my car after that to put the car up on jacks, then my check engine light was on so I figured it was the o2 sensor and I plugged it back in and it still had the check engine light so I checked it out with the obd tester and it said that it wasnt reading the o2 sensor so I cleared it out the car's computer and it worked fine.