Understand the difference
Why a code reader is only part of the answer
A diagnostic trouble code describes a condition detected by a control system. More than one underlying problem can lead to a similar result. A pending emissions code means the car has detected a condition that has not yet met all the criteria for a confirmed code. It still needs to be assessed alongside the symptoms and test results.
For example, an engine misfire may need checks of ignition, fueling, air leaks or mechanical condition. Replacing parts simply because their names appear in a code description can miss the cause.
Equally, no stored codes does not prove that a noise, vibration, leak or intermittent problem is absent. Your symptom history and appropriate physical testing still matter.