How to Understand Diagnostic Trouble Codes

There are two versions of On-Board Diagnostics (OBD):

OBD I (Vehicle model years 1988–1995)

In OBD I systems, when a fault condition occurs, the CEL stays on until the condition is resolved. This inhibits the driver from ignoring the problem. If the driver continues to ignore the problem, the vehicle will fail the next state-mandated smog check, which prevents the vehicle from being registered or "legally" sold. Some private sellers or dealers may attempt to sell a vehicle that has a code setting condition that is polluting the environment—this is illegal. Learn more >>

OBD II (Vehicle model years 1996–present)

OBD II is essentially OBD I with a new set of monitoring systems built on top. The monitors are bundles of software with increased testing capacity that either cycle constantly or when the system in question is in operational mode. Learn more >>

Included in this article are descriptions of:

  1. Continuous Component Monitor
    The Continuous Component Monitor is an updated version of OBD II and primarily focuses on pollution control. From the time the key is inserted into the ignition, the computer system boots and starts running self checks on all the powertrain components. This section focuses on cranking voltage, the Misfire Monitor system, and Fuel Trim Adaptation.

  2. Non-Continuous Component Monitor
    The Non-Continuous Component Monitors can vary between vehicle engine management systems, but there are several that are common to most vehicles. The term "non-continuous" means that the system and components are checked only under specific operating conditions. For example, in hot climates, there aren't any cold start conditions, so "stone-cold" startup monitoring will not occur at all in those places. This section focuses on common monitors, including the Oxygen Heater Monitor, Oxygen Sensor Monitor, and Catalyst Monitor.

  3. Basic Sections of an OBD II Catalytic Converter
    The next section focuses on the basic parts of an OBD II Catalytic Converter and the EVAP and EGR Monitors.

  4. Additional Monitors
    The last section of the article focuses on less common monitors like the Secondary Air System Monitor, Catalytic Heater Monitor, and Air Conditioning Monitor. It also has a section on the Mode 6 OBD II Data Stream for the Non-Continuous Monitors.

                                                                                           Next: A History of OBD I Trouble Codes >>