being able to pull computer codes helps, but too often the system which kicks the code is not the source of the problem:
Oxygen sensor starts failing and always reads lean ( computer pours more gas into engine => idle RPM drops => computer makes idle speed control motor push throttle and restore RPM => computer checks idle position of Throttle Position Sensor which is now out of tolerance, and a TPS error code is logged when the O2 sensor is kaput.
Still the codes give you somewhere to start.