Diagnostic trouble codes (DTCs) are 5-digit alphanumeric codes that identify the trouble area and help identify why the check engine light is on.
The 1st character is a letter that defines the main system where the problem ocurred. The 2nd is a number that identifies the type of code. The 3rd IDs the affected system, and the 4th and 5th characters define the section of the system that’s malfunctioning.
The most common DTC in 2017 was
P0420, which indicates Catalyst B1 Deterioration that often signifies a faulty oxygen sensor or cataltyic converter.