How to Retrieve OBD Trouble Codes
Repairing your car becomes less of a mystery once you learn how to retrieve OBD (On-Board Diagnostic) trouble codes. These codes are stored in the computer's memory when the Check Engine Light (CEL) or Malfunction Indicator Light (MIL) on your dashboard comes on.
These OBD trouble codes point you in the direction of the potential problem. For example, a trouble code may reveal an abnormal change in behavior in one or more of the engine circuits or sensors that affect the emission control system: A failed crankshaft position sensor (CKP), camshaft position (CMP) sensor, mass air flow (MAF) sensor, or any other sensor or its electrical circuit.
To diagnose complex electronic problems in newer cars, car technicians now need varied skills, special equipment, knowledge, and experience. Without their training and equipment, you'll have a hard time trying to solve every car emission problem just by pulling and interpreting diagnostic trouble codes.
On the other hand, you can deal with many engine systems yourself, especially if you get the help of your repair or service manual.
In this guide, you'll find ways to deal with trouble codes that point to simple repair jobs that may come your way, and suggestions on how to deal with the more difficult jobs.
Since 1982, when the California Air Resources Board required the introduction of OBD in all vehicles sold in the state, the OBD system has become more sophisticated.
In 1996, car manufacturers introduced a standardized OBD-II (second generation) system with more capacity and complexity to better control hydrocarbons (HC), carbon monoxide (CO), oxides of nitrogen (NOx), and evaporative emissions. Following in the same direction, modern vehicles now come equipped with computerized modules that use codes to identify potential failures related to the anti-lock brake system (ABS), supplemental restraint system (SRS), body control, and many other systems.
Retrieving Trouble Codes on First Generation (OBD-I) Systems
Before the appearance of the on-board diagnostic generation two system (OBD-II) in 1996, car manufacturers used different methods to retrieve data from the computer's memory and had their own code designations for each particular failure.
On Chrysler models, for example, you need to turn the key on and off three times, count the number of times the Check Engine flashes, and use the number to look up diagnostic codes. To access the codes on a General Motors model, you may use a jumper wire or paper clip to bridge the designated terminals on a diagnostic connector (usually found under the dashboard), and take note of the number of flashes from the Check Engine light. Getting the trouble codes on a Ford model is quite similar, except you use an analog voltmeter, and get the code number by counting the number of times the needle on the voltmeter deflects. The diagnostic connector on these models is usually located in the engine compartment.
Even so, on OBD-I systems, you may be able to use an auto code reader to retrieve the codes. Most auto parts stores carry simple code readers and scanners, and you can also buy them online. Before buying one, though, if you own a pre-1996 car model, consult the repair manual for your particular vehicle make and model, or a computer codes manual, so that you know exactly how to retrieve the codes. Most auto parts stores carry these manuals as well.
Your vehicle repair manual or the computer codes manual will help you interpret the trouble codes. Just keep in mind that OBD-I codes may differ from one vehicle model to the next, so make sure to look up the code for your particular vehicle make and model.
Retrieving Trouble Codes on Second Generation (OBD-II) Systems
Today, vehicles come equipped with complex engine control systems that require expensive equipment to access and analyze all the data stored in a car's computer during a failure. The good news is that the trouble codes you are most interested in on OBD-II systems have been standardized, so they have the same meaning across manufacturers' vehicle models. And you can buy relatively inexpensive aftermarket scan tools that let you retrieve some of the diagnostic codes you can use to troubleshoot simple engine control system problems and do maintenance work.
A scan tool, along with your repair manual, can help you save hundreds of dollars a year in simple repairs. And the more you learn about your car systems and how they work, you'll gain experience to do more maintenance and other easy repairs on your own car.
On 1996 and newer vehicle models, you can use a code reader or scan tool to retrieve the codes. Connect the tool to the data link connector found underneath the dashboard, on the driver's side. Then, follow the instructions that come with your tool, or in your repair manual, to retrieve the codes.
Your vehicle repair manual, or the computer codes manual, will help you interpret the trouble codes.
Why Your Computer Stores an OBD Code
Your car's computer constantly receives information from sensors and sends information to actuators as electrical signals. The number of systems the computer “talks” to depends on your vehicle make and model.
To know that systems are working properly, the computer compares the received information with the data stored in memory. If one of the received signals doesn’t match a preconfigured value or range of values, the computer may store a trouble code and turn the engine light on or wait for another drive cycle to confirm a potential problem. This depends on the type of failure it detects. But once your car's computer confirms a potential failure, it'll save the specific trouble code(s) and alert you.
You then may retrieve the code(s). As described above, you may get the code through the data link connector using a piece of wire, a voltmeter; by turning the ignition key; through a code reader or scan tool. The best way to proceed is to follow the instructions in your vehicle repair manual for your particular make and model. If you can retrieve the codes using a code reader or scan tool, follow closely the instructions that come with your tool.
Deciphering Diagnostic Codes
A typical diagnostic trouble code is made up of a letter and four numbers. It may look something like this: P0136.
P: The letter at the beginning of the code points to the overall function of the system involved in the problem (powertrain, chassis, body, etc.). In this case, the powertrain.
0: The next digit could be a number assigned to a standard type of code, like a 0; or a number 1, referring to a particular car manufacturer code.
1: The next number tells you the function of the system that registered the problem. In our example, it is telling you that the system involved measures the fuel and air content going into the engine.
36: Finally, the next 2 digits point you to the component or circuit in question and the type of problem it found. In this case, it tells you the downstream heated oxygen sensor in the Bank 1 section has either detected a problem with the air/fuel mixture; or the component itself--or its circuit--has stopped working.
Where to Find Information on Diagnostic Trouble Codes (DTCs)
- Your vehicle's repair manual
- Your scan tool's manual
- Online car repair forums
- The Web
Where to Find Diagnostic Trouble Codes Information
Armed with the trouble code, you can find out the meaning of the code by consulting your vehicle repair manual, or a computer code retrieval tool manual. You also can find other code interpretation sources by searching online or by posting a question in one of the many car repair forums on the Web.
If you used a code reader or scan tool to retrieve the code, chances are your tool’s manual will have a list of codes and their meaning. In addition, your local auto parts store or dealer’s service department may help you as well.
Once you know the meaning of your trouble code(s), you may proceed to troubleshoot the component itself and its circuit. For this, you may want to consult your vehicle repair manual. The manual not only tells you how the different systems and components in your vehicle operate, but also helps you locate and troubleshoot many of these components and systems following a step by step procedure.
You can also find help from car technicians online in one or more of the car repair forums on the Web.
One thing you should keep in mind when dealing with trouble codes: Any time the computer stores a diagnostic code, it points you toward the system or potential part involved in the failure, but you still have to troubleshoot that particular part or circuit to find out which component is at fault, if any. Sometimes, a trouble code that seems to come from one system has been triggered by another component in a related system.
If you feel you can't deal with a particular repair, you still can use the DTC information to help a shop narrow down a problem and make a repair estimate. Request estimates from different shops in your area so you can make an informed decision.
With the help of a simple code reader or scan tool, you can do a lot to repair and maintain your own vehicle any time you see the Check Engine light come on.
Vehicle Repair Manuals
Haynes makes good aftermarket manuals for a wide range of vehicles, including trouble codes and much other information for DIY diagnosis. Find the manual for your exact make and model.