OBD, or on board diagnostics, was first introduced by General Motors in 1981. The purpose of "OBD" was to monitor the emission control system in the car. When the computer system of the car sees a fault in the emission control system, three things are supposed to happen. First, it would set a warning light on the dashboard, to inform the driver that a problem existed. Second, to set a code in the computer. Third to record that code in the computer's memory, that can be later retrieved by a technician for diagnosis and repair.

This system worked so well, in 1986 California mandated that all cars sold in the state be equipped with "OBD". This then became an industry standard throughout the nation, and all cars sold in the nation had some form of "OBD"

This first version of"OBD" had a lot of shortcomings. First, it only covered the engine emission system. The fuel tank vapors were not monitored. The exhaust emissions were not measured. And only devices specifically installed for emission control were monitored. Second, there was no standardization throughout the industry. Each manufacturer had a different term for the warning light that was illuminated when a fault was determined. GM called it a "check engine" or "service engine" light. Chrysler called it a "power loss"light. Ford called it an "engine" light. Most foreign cars called it a "check engine"light. This was not only confusing to the technician, but also to the motorist. Many motorists upon seeing the "service engine" light illuminated, brought their car to a repair facility and either asked for an oil change or tune-up, expecting the light to go out. Needless to say, this did not happen, and after spending unnecessary money on service work, the system then had to be diagnosed and repaired. The coding system for each manufacturer was also different making diagnosis much tougher.

The clean air act of 1990 mandated that beginning with the 1996 model year, all cars sold in the U.S. be equipped with a new version of on board diagnostics This system became known as OBD II. The manufacturers beat the deadline and almost all cars were equipped with OBD II in the 1995 model year. If your car is a 1995 model or newer, chances are it is equipped with OBD II.

Among the many differences between OBD and OBD II, was the standardization of the system. All dashboard warning lights now say "check engine", usually with a picture of an engine with the word "check" across it. The coding system is now standard. There are now over 400 possible trouble codes that can be stored in the system. All causes of excessive are now monitored. If the gas cap is left loose and vapors are escaping from the gas tank, the "check engine" light will be illuminated and a code will be set. It is extremely important now that the engine be shut off when refueling the vehicle. Another big difference between the systems is that with OBD when a fault is seen the warning light is illuminated and a code set. The warning light will then go out when the fault is no longer seen, but the code will be set and retained in the computer's memory. In OBD II systems the light does not go out until the fault is repaired and reset by the technician. This can create two problems for the motorist, first, if the warning light is set because of a loose gas cap, it will not go out when the cap is tightened. The car will have to be brought to a service facility to reset the light, at a cost to the motorist. Second the danger exists that when the car is brought into a repair facility, the technician might reset the light without actually repairing the fault. In this case the light will come back on again.

It is important that the motorist be aware of the "check engine" light, as well as all other dashboard warning lights and what they mean. This knowledge will help eliminate unnecessary costs due to unnecessary repairs. When any dashboard warning light comes on, check the owners manual before bringing the car to a repair facility.

OBD II Generic vs. OBD II manufacturer specific codes

Breakdown of a OBD II generic code vs. OBD II manufacturer code:



Diagnostic trouble codes are alphanumeric codes that are used to identify a problem that is present on any of the systems that are monitored by the on-board computer (ECM or PCM).



EOBD and OBD II diagnostic codes are composed of five characters; one letter followed by 4 digits, e.g. P0101.



The first character is a letter. It identifies the "main system" where the fault originated. This will be one of the following systems:

Letter System

B Body

C Chassis

P Powertrain = "Check engine"

U Network (UART)



The second character is a numeric digit. This identifies the type of code.

Code type Explanation

Generic

(normally P0xxx) The definition for the code is defined in the EOBD / OBDII standard and will be the same for all manufacturers.

Manufacturer-specific

(normally P1xxx) Where manufacturers feel that a code is not available within the generic list, they can add their own codes. The definitions for these are set by the manufacturer.



In general, codes that begin with P0 are Generic codes, whereas codes that begin with P1 are manufacturer-specific. Additional code group are however available to allow for expansion of these code lists.



The full breakdown of the code groups is shown below:

Powertrain codes Body codes

P0xxx - Generic B0xxx - Generic

P1xxx - Manufacturer-specific B1xxx - Manufacturer-specific

P2xxx - Generic B2xxx - Manufacturer-specific

P30xx-P33xx - Manufacturer-specific B3xxx - Generic

P34xx-P39xx - Generic

Chassis codes Network Communication codes

C0xxx - Generic U0xxx - Generic

C1xxx - Manufacturer-specific U1xxx - Manufacturer-specific

C2xxx - Manufacturer-specific U2xxx - Manufacturer-specific

C3xxx - Generic U3xxx - Generic



The third digit defines the specific system or sub-system within the car where the problem is located:

Third digit System or sub-system

1 Fuel and Air Metering

2 Fuel and Air Metering (injector circuit malfunction only)

3 Ignition System or Misfire

4 Auxiliary Emission Control System

5 Vehicle Speed Control and Idle Control System

6 Computer Output Circuits

7 Transmission

8 Transmission



It should be noted that manufacturers are not required to follow the generic numbering system above for their manufacturer-specific codes (many do appear to follow these though).





The fourth and fifth digits identify the section of the system causing the fault. There is no detailed list of the components indicated by these digits, as each system or sub-system will have many different components.