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On Board Diagnostic Systems OBD2

All vehicles sold in the US for the model year 1996 and newer are required to meet the On Board Diagnostics 2nd Generation (OBD2) standard. The standard requires that the on board computer monitor and perform diagnostic tests to ensure that vehicle emissions do not exceed the legal limit. The computer is often referred to as the Powertrain Control Module (PCM). The Malfunction Indicator Lamp (MIL) will come on if a system or component fails or deteriorates to the extent that the vehicle emissions could rise to 1.5 times the US Federal limit. The European Union has a version of OBD called EOBD. It is similar to OBD2 in many respects but there are some differences.

All OBD2 compliant vehicles have the same 16 pin Data Link Connector (DLC) under the dash and an emissions control label under the hood stating that the vehicle is OBD2 compliant. Many jurisdictions are moving away from traditional tailpipe testing to OBD2 testing to determine whether a vehicle complies wth emission regulations.

OBD2 systems monitor many areas of engine operation. Three areas are monitored continuously while other areas are monitored only when specific conditions are detected. Activating these non-continuous monitors may require that the vehicle be driven at a certain speed for a certain length of time, specific engine temperature conditions and acceleration/deceleration cycles among others. Not all of these monitors, sometimes referred to as Inspection and Maintenance (I/M) Readiness monitors, apply to every vehicle.

Continuous Monitors
Engine misfire, fuel system operation and a range of engine components are continuously monitored. This is done since a malfunction in any of these systems could cause vehicle emissions to exceed the allowable level. The components monitored may include sensors dealing with vehicle speed, throttle position, coolant temperaure, crank angle, engine knock, fuel composition and many others. When a malfunction occurs, a fault code is stored and depending on the potential effect on vehicle emissions, the MIL light may be turned on.

Non-continuous monitors

The non-continuous monitors are catalyst, heated catalyst, oxygen sensor, oxygen sensor heater, evaporative system, secondary air system, air conditioning system refrigerant and exhaust gas recirculation system. Thermostat and positive crankcase ventilation monitoring were added in year 2000 and 2002 respectively.

It is important to understand the meaning of some of the terms commonly used when describing OBD2 systems. Here are a few.

Generic Diagnostic Trouble Code (DTC)

These are the codes that are required by law on all OBD2 equipped vehicles. A generic code has the same meaning regardless of the make or model of car.
Enhanced DTC
These are codes generated by a manufacturer to provide additional information about system faults that may or may not be emission related. An enhanced code may have a different meaning from manufacturer to manufacturer.
Pending DTC
Some diagnostic trouble codes are only stored if the fault occurs a certain number of times. Until that happens, the code is regarded as pending. Should the fault not reoccur within a set period of time, the DTC will be cleared.
Freeze Frame Data
When an emissions related fault occurs, the system not only sets a code but also records a snapshot of vehicle operating parameters to aid in identifying the problem. This block of values is referred to as Freeze Frame Data and may include engine rpm, vehicle speed, air flow, engine load, fuel pressure, fuel trim value, engine coolant temperature, intake manifold pressure and open or closed loop status.

Drive Cycle

This consists of a warm up cycle where from a cold start, the vehicle is driven long enough to raise the coolant temperature by at least 40 degrees F. Here is an example of a drive cycle for Ford cars.

There are many free online resources for those interested in learning more about OBD2 systems. Some of these provide general information while others deal with the subject in much more detail. There are also forums where one can get questions answered.