| When the "check engine" or "service engine soon" | | | | 4 = Auxiliary Emission Control System |
| light comes on, it doesn't tell you what or where the | | | | 5 = Vehicle Speed Control and Idle Control System |
| trouble is, only that there's trouble. You've hooked up | | | | 6 = Computer Output Circuits |
| your scanner to the vehicle computer and retrieved | | | | 7 = Transmission |
| the OBDII diagnostic code. What does it mean? | | | | 8 = Transmission |
| The "OBD" of OBDII - sometimes written OBD-II or | | | | So our example trouble code P0304 indicates a |
| just plain OBD2 - stands for On-Board Diagnostics. | | | | problem in the powertrain. It is a generic code for |
| The "II" is the next generation of emissions | | | | trouble with the ignition system or a misfire. |
| standards and codes for all vehicles sold in the U.S. | | | | The fourth and fifth numbers of the code |
| from 1996 to the present, domestic and imports. | | | | correspond to the section of the system causing the |
| The OBD2 system is primarily for emissions control. | | | | trouble. The list of all these sections is long, but you |
| Its basic components are the catalytic converter and | | | | can see how the final "04" in our example points to a |
| strategically-placed oxygen sensors. These as well as | | | | misfire in cylinder number four. |
| everything in the vehicle having to do with engine | | | | Most OBDII diagnostic scanners will come with a |
| performance and emissions control are continuously | | | | code library of specific code meanings. A list may also |
| monitored by the vehicle's on-board computer | | | | be found on-line by Googling "obd2 codes list". |
| system. | | | | You will sometimes find a reference to "Bank 1" or |
| The "check engine" or "service engine soon" light is | | | | "Bank 2" in the code explanation. These banks are |
| the signal that there is a problem with the vehicle's | | | | generally meant for "V-type" engines. Bank 1 refers |
| emissions. The computer has assigned a trouble code | | | | to the side of the engine with the number 1 cylinder |
| to the problem and turned on the trouble light - | | | | (odd-numbered cylinders). Bank 2 refers to the side |
| technically called the Malfunction Indicator Lamp (MIL). | | | | of the engine with the number 2 cylinder |
| When you retrieve the information from the | | | | (even-numbered cylinders). |
| computer via an OBDII diagnostic scanner, it doesn't | | | | A misfire is a "one-trip" or "type A" problem that by |
| tell you the problem directly, such as "timing too | | | | itself will not turn on the MIL unless it is severe |
| slow" or "misfire in cylinder number four". What you | | | | enough to damage the catalytic converter. A severe |
| get is a five-digit alpha-numeric code such as "P0304". | | | | misfire will not only turn on the MIL but will signal it to |
| The first digit is a letter corresponding to the main | | | | flash at one-second intervals. |
| system causing the trouble code: | | | | Other problems are usually "two-trip" or "type B" |
| B = Body | | | | problems. When the computer first detects a |
| C = Chassis | | | | two-trip problem, it stores the trouble code as |
| P = Powertrain | | | | "pending". If on the next driving trip the problem has |
| U = Network | | | | passed, the pending code is erased. But if the |
| The next four digits are all numbers. There is never a | | | | problem is still there on consecutive trips, the |
| letter "O" in the OBDII diagnostic code. It is the | | | | computer will turn on the MIL, alerting the driver to a |
| numeric digit zero ("0"). | | | | problem. |
| The second digit corresponds to the type of code, | | | | The MIL can be persistent. Once on, it will stay on |
| whether it is the generic standard applying to all | | | | until the problem is resolved for three driving trips. |
| OBDII-compliant vehicles, or a manufacturer-specific | | | | However, though the light may go out, the codes |
| code. | | | | remain in the computer memory for 40-80 trips |
| 0 = Generic codes | | | | depending on the problem. |
| 1 = Manufacturer-specific codes | | | | Trouble codes remaining in the computer memory will |
| 2 = Includes both generic and manufacturer-specific | | | | cause a failed emissions inspection whether the |
| codes | | | | "check engine" light is on or not. |
| 3 = Includes both generic and manufacturer-specific | | | | Of course, your diagnostic scanner can turn off the |
| codes | | | | MIL and erase the codes from the computer's |
| The third number corresponds to the sub-system | | | | memory. This will not do much good, however, if the |
| where the problem lies. | | | | problem recurs after two driving trips and |
| 1 = Fuel and Air Metering | | | | regenerates the trouble codes. The information from |
| 2 = Fuel and Air Metering (injector circuit malfunction | | | | the scanner should be used to locate and fix the |
| only) | | | | problem, not just turn off the MIL and erase the |
| 3 = Ignition System or Misfire | | | | codes. |