OBD-2 Check Engine Code

If your vehicle was manufactured after 1996, it contains an On Board Diagnostic (OBD) computer that captures information about how it's running. OBD II is version 2.0 of the standard for communicating this information.

The data tracked by the OBD II system was originally intended to monitor the engine's emissions and track down problems that caused cars to pollute more than normal. Today, however, manufacturers have extended the standard to contain a great deal of data about problems and performance. OBD II data is what causes your car's "check engine" light to go on when there is a problem, and it is your mechanic's tool of first recourse when you bring the car in with symptoms that have no obvious cause.

Since the data's transmission format and content are standardized, a number of third parties have developed hardware to detect and display these codes. Some of these devices hook up to laptops, which means you can display and catpure this data.

The OBD II port allows your car to report three kinds of information: Diagnostic Trouble Codes (DTCs), real-time data, and freeze frame data.

DTCs are simply error codes that can be looked up to determine what problem your car is experiencing. For example, the DTC P0302 means "cylinder 2 misfire detected". If the condition that caused the DTC persists, the car's computer will turn on the "check engine" light.

Real-time data is the raw sensor data reported to the OBD computer. This data can be helpful for troubleshooting problems and monitoring engine performance.

Freeze frame data is a snapshot of the real-time sensor feeds at the time of a DTC condition. An auto mechanic can use this data to figure out what was going on at the time your car's "check engine" light went on.

Within the OBD II standard, there are several protocols for transferring data from the car to a diagnostic device.

Name	Speed	Used by
ISO 9141	10 Kbits/second	most Asian and European manufacturers
J1850 PWM (Pulse Width Modulation)	100 Kbits/s	Ford, Mazda
J1850 VPW (Variable Pulse Width)	100 Kbits/s	primarily GM
CAN (Controller Area Network)	250 Kbit/s or 500 Kbit/s	newer vehicles

The CAN standard is newly emerging. It will be required on all new vehicles by 2008, and it will help various computer systems within your car to communicate. For example, your GPS system could talk to your OBD system or your DVD player.