What is OBD II and how does it work?

What is OBD

You're driving along the road one day, when out of nowhere, your car decides to get your attention with that cheery "ding" noise. You look at your dash and the "Check Engine" light is shining. "Oh no," you think, "what does this mean? What's the problem and how can I solve it?"

The check engine light is the signaling device for your vehicle's onboard diagnostics or OBD system. The Environmental Protection Agency requires vehicles to have onboard diagnostic systems to warn drivers about problems that affect their emissions. The system also records other information that can make it easier for you or a mechanic to find and fix problems with your car.

The first onboard diagnostic systems were proprietary to different carmakers. In 1991, California required that new cars have some sort of OBD system. Different manufacturers used different systems. These are usually referred to as OBD I systems today. In 1996, the US government made a standardized system mandatory for new cars. This system is OBDII. After 1996, all vehicles sold in the US use the same system, with the same connector, to be read with the same tool, with the same diagnostic codes. That makes diagnosing car problems easier across the board. We'll walk you through how the OBDII system works, and how you can read the codes it uses.

How OBDII Works

There are many sensors throughout your car: oxygen sensors, engine knock sensors, manifold pressure sensors and so on and so on. Each one of these sensors sends a signal to your car's computer the Engine Control Unit (ECU). The ECU uses that information to adjust different elements of your engine operation, the fuel injection or the spark timing for example.

If the information that the ECU gets from one of its sensors is too far out of whack, it saves a code called a Diagnostic Trouble Code (DTC). It also sends a signal to you check engine light. If the light comes on and stays on, then you have a minor problem. You can still drive the car until you get it looked at. If the light blinks on and off, your problem is urgent and might damage the engine. Engine knocking is an example of a problem that might give you this signal. If your check engine light is blinking you should avoid driving your car as much as you can, and avoid driving aggressively until you fix the problem.

How to Make the Diagnosis

You can find what code or codes your ECU has saved by using a diagnostic scan tool, sometimes just called a scan tool, code reader, or scanner. The scanner connects to a port usually found underneath your dashboard on the driver side. These ports are standardized so any OBDII scanner should be able to read codes from any OBDII car. The scanner can then check for saved DTC and show you the codes on the scanner's screen. Most scanners can also perform other functions like letting you check the measurements taken by your various sensors in real time. If you work on cars frequently, a scanner can be an invaluable tool.

Once you have the code, how do you figure out what the problem is? Some scan tools will give you the code along with a short description. Most of the codes do follow a system, though, that can be interpreted with a little bit of know-how.

The code is made up of a letter followed by four digits. Each one of these parts carries a specific meaning. The letter tells you what overall section of the car the code is coming from. P stands for the powertrain, the engine and transmission. B stands for the body. C stands for the chassis, including the brakes and suspension. U codes are for problems with the computer itself.

The first digit tells you whether the code is general or manufacturer specific. Most of the codes are universal and apply across manufacturers. These codes are standardized by the Society of automotive engineers. Manufacturers might add their own codes for their particular systems. Standardized codes have a 0 or 3 as their first digit, while manufacturer specific codes have a 1 or 2. The second digit further specifies which particular sub-system is affected.

The breakdown of the second digit varies from one system to another. This chart brakes down all the subsystems by the first three characters

P01 Fuel and Air Metering
P02 Injector Circuit
P03 Ignition System
P04 Auxillary Emissions Control
P05 Vehicle Speed Control and Idle Control
P06 Computer Output Circuits
P07 Transmission
P08 Transmission
B00 Body, including airbags and seatbelts
C00 ABS
C01 Brake Hydraulics
C02 Wheel Speed Sensors and Traction Control
C03 4WD
C04 Steering
C05 Steering
C06 Suspension and Leveling
C07 Tire Pressure
C08 Suspension and Leveling
U00 Communication Bus
U01 Lost Communication With Sensor
U02 Lost Communication With Sensor
U03 Software Incompatibility
U04 Invalid Data Received

The final two digits signify the specific problem. You can then check this code against information from the SAE to determine where your problem lies.

Manufacturer specific codes might not follow these rules, past the letter and the first digit. If you get a manufacturer specific code you will have to consult a service manual or some other manufacturer-specific resource to determine the cause of your problem.

Hopefully, with this information, the next time that pesky check engine light comes on, you'll be able to figure out what it is trying to tell you. 

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