Originally Posted by Kaldaien

If you have an Android cell phone, ~$15 will get you an OBD-II Bluetooth transmitter and you can pair that with your phone and use an app called Torque. Drive the car around and log the ECU data with Torque. It's probably every bit as good as having a worthless technician in your car with their data logger hooked up, plus you have a new toy for when your car's working correctly

There's probably an equivalent tool for iOS, but I have no experience with iPhone.